Tatsumi, as the main protagonist, has some sort of relationship with almost every character in the series. For this reason, only those with a profound impact on him will be listed.
Sayo and Ieyasu: Although only appearing briefly in the manga (and mostly in flashbacks) it has been seen that Tatsumi is very close with his childhood friends and that they have a large impact on Tatsumi's decision of whether to side with the Empire or the Revolutionary Army. The death of his friends was Tatsumi's initial drive in becoming an assassin and wanting to change the corrupted government.
Leone: Tatsumi and Leone have a close relationship due to the fact that Leone is the reason that Tatsumi was inaugurated by the Night Raid in the first place. She also was the first person he met while at the capital and where she swindled him out of his money. After a short time, the two of them become close friends and are seen to laughing and spending time together. Leone seems to develop some romantic feelings for Tatsumi, calling him "cute" on several occasions, constantly shoving him into her breasts (which could arguably be for comic relief) and at one point, Leone literally "marks" him as her future man. She has also stated that she would not lose Tatsumi to Esdeath.
Bulat: Tatsumi sees Bulat as a mentor and older brother, calling him "aniki". Soon after their first meeting, they become very close and although Tatsumi did not agree with Bulat's attractions they form a strong brotherly bond. When Bulat is later killed by Liver, Tatsumi is devastated and trains hard every day to live up to Bulat's expectations and master the Teigu entrusted to him. Bulat's teachings have made a significant and permanent impact on Tatsumi as an assassin, both physically and emotionally. Thanks to Bulat, Tatsumi is now better able to control his emotions regardless the situation, shown when he was captured by Esdeath and had to remain calm in order to keep his identity a secret.
Akame: While at first feeling very uncomfortable around Akame due to her rather distant behavior and the fact that she nearly killed him when they first met, Tatsumi later comes to see Akame as a close fiend after he realizes she is a very caring person who hates to see her friends hurt. Akame and Tatsumi share a very close relationship and even promise to kill each other should it ever be necessary. He also promises to her after Sheele's death that he will not allow Akame to experience the pain of losing more comrades, saying that he will definitely survive. Akame later helps him keep his promise by killing the Tyrant.
Mine: When first teamed up with Mine, Tatsumi did not get along with her. She mocked him frequently about his worth to the Night Raid, before he even joined them. She also considered him not being cut of the same "professional cloth" as the others. After the success of their first mission together she started to warm up to him, but this did not stop her from continously teasing him. During the mission to assassinate Bolic, she and Tatsumi functioned as partners to scout the outer areas of the city where the Jaegers weren't likely to appear. They started bickering but their relationship was confused for a romantic one by the leader of the Way of Peace. Mine has also trusted Tatsumi to deal with the last Rakshasa demon and was visibly thankful after he saved her life from Seryu's final attempt to kill her using a suicide bomb. Following a confession of love from Mine, he accepts her feelings and the pair form a relationship. Over time he also developed feelings for Mine, stating that she was right when she said that he would be "Head over Heels" for her. After Mine was rendered comatose by using all of her power to kill Budo, Tatsumi told her comatose body that once the revolution was over he wanted to marry her, while also declaring his love for her. True to his word, Mine eventually recovered from her comatose state and reaffirmed their love for each other, despite Tatsumi's permanent state as a dragon. It is also revealed that Mine was pregnant with Tatsumi's child. The two retired from duty and retreated far from the Empire where they lived happily together with their children.
Sheele: Although Tatsumi was at first put off by Sheele's blunt death threat, he quickly warmed up to her and she comforted him while he was mourning the deaths of his two friends, Sayo and Ieyasu. After her death in battle, Tatsumi was overcome with rage and wanted to seek revenge, but was stopped by the other members of Night Raid. Although the time that they spent together was short, they developed a close friendship with Sheele taking on somewhat of a motherly role. She also developed feelings for Tatsumi and her last thoughts were about him.
Lubbock: Lubbock and Tatsumi have something of a comical rivalry, with Lubbock describing Tatsumi as a "lady killer", much to Lubbock's chargin. Still, the two see each other as comrades and worry about each others' well-being. Lubbock sometimes takes advantage of Tatsumi's innocence, such as sending him to peep on Chelsea while she is bathing in order to assess the danger. The extent of their friendship was shown when the two were captured, and despite Syura's attempts to break him by making Lubbock see that Tatsumi is the reason he was going trough so much suffering, Lubbock revealed that he never once wavered and saw Tatsumi as a true friend and comrade. When Lubbock was killed, Tatsumi showed great anger and grief, and blamed himself for his death, though Najeda assured him that Lubbock wouldn't want him to beat himself up over it.
Chelsea: Although Chelsea angered most of Night Raid (including Tatsumi) when she declared them "too soft", Tatsumi came to realize that she meant no harm and was only worried about the team's well-being. He respected her abilities and knowledge of battle and became friends with her after a short period of time. It is hinted that Chelsea may have developed romantic feelings for Tatsumi as she notices that her attention is mostly on him (causing her to blush), and even blushed whenever she was around him. In fact, her final thoughts were of him, just like those of Sheele. Tatsumi was devastated by her death as he considered her a valuable friend.
Susanoo: Although only having recently joined Night Raid, Tatsumi instantly warms up to him, referring to him as "Suu" like most of the other members. Susanoo has been seen training Tatsumi during the one month time-skip in order for Tatsumi to hone his skills. Tatsumi demonstrates a fair amount of emotion after Susanoo's demise.
Esdeath: The sadistic general discovers Tatsumi when he competes in (and wins) the fighting tournament that she had set up. Completing the general's criteria with his pure smile, Esdeath falls in love with him at first sight. Soon after his first match, she instantly declares Tatsumi as her lover and drags him off with her. Seeing her affections for him as a chance to possibly convince her to change sides, his first attempt ends unsuccessful, as she declared that she would never understand the words of the weak, and that she would change Tatsumi, though Tatsumi retorted that he wouldn't change. After meeting her a second time, and then getting trapped in an island with her, Tatsumi spent some time with her, and the two seemed to have fun together, with Tatsumi still thinking that he could change her. However, after realizing that despite how her tribe was killed by another tribe, and how she had a Danger Beast blood within her, Esdeath was cruel to her very core, and sadly accepting that he could never convince her to change. When he found a chance to escape and leave her on the island, Tatsumi hesitated in abandoning her, and called out to her, before he made his escape, repaying his debt to her. Afterwards, Tatsumi accepted that when they met again, they would certainly be enemies, though he showed a sign of sadness from that. After the revolution started and he started going out with Mine, Tatsumi met Esdeath a third time after being captured by her once his identity as a member of Night Raid was exposed. Despite Esdeath's attempts to show affection to him once more, and a desperate attempt to try and convince him to join her, Tatsumi rejected her completely, and showed anger towards her, especially when told by her that Lubo was killed. After his execution was stopped and he escaped with his comrades, Tatsumi knew that the next time he faced against Esdeath, she would have every intention to kill him and with the final battle approaching he would put everything on the line to defeat her when the time came.
Wave: The two became good friends in the couple of days after Tatsumi was introduced to the Jaegars. When the two were partnered up in Chapter 18, they got to know each other a little. Wave then told him that he and Tatsumi were somehow similar to each other, being of their situations in their factions. They got along pretty well and were a good team when defeating a danger beast. It was revealed that the two have similar Teigus. However, both of them didn't realize that they were fighting against each other because their armored Teigus covered up their appearances. Tatsumi himself admitted that Wave is not evil, but stated that he is willing to kill him if he stood in his way.
Bols: Tatsumi first meet Bols when he was dragged by Esdeath into the Jaegars HQ and there he learned that Bols was married for six years. The two didn't have any interaction at all other than this. When the order came to assassinate Bols Tatsumi felt uneasy about it possibly due to him being married. However, Tatsumi did participate in the attempt at Bols life.
Seryu Ubiquitous: Originally, he and Seryu got along well despite their short time they spent together, however all that changed when both learned that they operated in enemy groups. Tatsumi held a grudge towards her for killing Sheele and had to hold back his anger not to expose himself. Seryu vowed to kill him just for being a member of Night Raid to avenge Ogre. When she eventually discovers that he is a member of Night Raid, she states that he betrayed the high regard in which Esdeath held him. She never learned that Tatsumi was the one who killed her mentor.