Grim Jr, being the main character of Grim Tales has interacted or connection with everyone.
Minnie is the maternal half-sister of Grim Jr. They have slightly complex relationship. He was first jealous of her because she was always in the spotlight and even yelled at her for saving his life, which in turn got her killed. After receiving her powers, Grim Jr. searched for her soul and during it, he learned how much she really loved him. After regaining her soul back, his jealousy was gone, thus starting a strong bond. He is aware of her more than just familiar love for him, and as revealed in "Frustration" and "Minnie needs therapy" although he doesn't loves her the same way. As shown in later on chapters, both have come to terms with Minnie's feelings and they seem to have grown very close despite of it, but Grim Jr. still doesn't love her the same way. He is also very protective of her now. After seeing Junior saves Mimi and paying less attention to herself, she becomes more cold and mean toward Junior, going as far as to step on his muffins and insult their taste, to Junior's confusion, but he doesn't seem to take it too hard.
Grim is the father of Grim Jr. and although not fully explored yet, Grim Jr. seems to love his father and has respect for him. Grim cares dearly for his son and wants no harm to become of him. Although Grim is technically the father of all aborted children and therefore had to get someone else to impregate Mandy so she could abort it, he loves Grim Jr. very much and claims him as his only son.
Mandy is the mother of Grim Jr. and although she seems cold and heartless, she seems to care and geuniely love her children, albeit in her own way. Grim Jr. is very scared of his mom, yet ironically it is her name he calls for whenever he is in trouble(before he got demonic abilities). Even though he holds great fear and respect for Mandy, he did stand up to her when she was harming Mimi, showing that when the time calls for it, he casts aside his fear of her to defend a friend. He did feel bad about doing it though afterward. He loves his mom a lot as well, getting into a fight with his Nergal side when it insulted her numerous times. He seems to be her favorite child, if his nergaling saying is true. She is also apparently proud of him for standing up to her for Mimi's sake, seeing how she she easily brushed the matter and left instead of continuing to assault him, while complimenting that he has some guts.
Manny is the half-brother of Grim Jr. they have the same mother. Manny has the same personality as Grim Jr. had when he was younger. He strives, just like Grim Jr., for strength but doesn't pay any attention to techniques (although Manny seems to have even more trouble with that than Grim Jr.). He sees Grim Jr. as his big brother and likewise.
Daniela is the half-sister of Grim Jr. they also have the same mother. Daniela doesn't have a very good relationship with Grim Jr. but did shown that she loves him (in a way a sibling would). She resembles Minnie very much, mostly in her way of speaking and attitude.
Jack is the other uncle of Grim Jr. When introduced Grim Jr., Jack scared him. But he seems to be on good terms with his Pumpkin King of an uncle.
Sally is the aunt of Grim Jr. and he is on good terms with her. When she asked him to dance with her, he(trying to outshine his sister) started breakdancing, but tripped over Minnie's foot in the process, which caused everyone to laugh and him run off.
The Nergaling is the unnamed Nergaling that resides inside of Grim Junior. He first was the Nergaling of Minimandy until Minnie gave her powers to Junior. After receiving the powers, Junior was in a state of grief and couldn't handle the power of the Nergaling. This caused it to merge with the Pumpkinator and destory all of Halloween Town. Later Junior and him have seem to have become friends. Junior now can control the Nergaling, and also talks a lot with him. As he is his only friend in the castle. They do argue a lot, but are good friends.
Nergal Jr. is the Godfather of Grim Jr. and who he adores and looks up to the most. He wanted to be so much like Nergal Jr. but because of Minnie, he started seeing less and less of him which made him angry. Although the relationship hasn't been fully explored, it would seem all is good now.
Nergal is the Grandfather of MinnieMandy and the father of Nergal Jr. Because his essence is in every Nergal demon, he is inside of Grim Jr. He often helps Grim Jr. with his powers and is there to give advice on some siutations. They have a pretty good relationship right so far.
Lord Pain is the first guardian of the army, but also the nanny of both Grim Jr. and Minimandy. Junior and him seem to get along very well. He cares about Pain, as seen when he was concerned about him when Mimi defeated him. Also Pain didn't mind to make Mimi's favorite dishes although she had almost killed him. It seems that he did it because it would make Junior happy.
Zero and Grim Jr. are basic friends and have interacted for a limited span of time. When he first met Zero, he played catch by taking one of his rib bones and throwing it. After he got kidnapped it was Zero who went and got help from Minnie.
The relationship between Grim Junior and Mimi has yet to be given a confirming name. At first Mimi was introduced as an enemy of of Junior. However when HIM treatened to kill Jeff for betrayal, she teleported Jeff and Junior to Limbo. So it seemed that she didn't want him to get hurt. When Junior said goodbye to her on Limbo they shared a moment, only to be ruined my Mimi who pulled Junior's cheeks instead. They met later again in the Castle of Grim where he wanted to calm Mimi down and prevent her from fighting Minnie, but it didn't help and resulted him being pinned down to the ground by her instead. After Junior watched her worst memory, Mandy had the intention to kill her. Junior defended Mimi, probably because he developed feelings for her, and fought his mother. Having proved worth to his mother he was allowed to keep her as a "pet" in the castle. He cares for Mimi during this time. He even wanted to save her when she almost walked off the castle. Junior often blushes when getting into an intense situation with Mimi, or if she displays an "Ecchi" scene.
Sadako, Samara Morgen in the English version and also know as "the girl from the well". She was Grim Jr.'s crush in the past, and was featured with him in art and mentioned by Minnie.
HIM is the biggest enemy of Grim Jr. HIM knows everything there is to know about Grim Jr. while Junior only knows something of his past. He is willing to defeat HIM to save the Underworld and free Mimi from his tyrany. They have fought each other once, but ended in a draw when Mimi teleported away to Limbo.
Oogie Boogie & allies
Oogie Boogie was the first enemy that Grim Junior encountered during his journey. After being tricked by his fake friends Lock , Shock and Barrel , Oogie tried to obtain his Reaper powers by cracking his head. He however was saved by Minnie, by then again was killed by the Pumpkinator . When Junior's Nergaling killed and merged with the Pumpkinator, he killed Oogie Boogie along with his accomplices.
Redeemer first despise Junior for his jealousy to Minnie and took her with him but lost her so he teamed up with Grim Jr. to save her. During the course of their adventure the angelic warrior told Jr. to stop looking at things with just his eyes for they can only show so much. The two seemed to have reconciled and are now on good terms. Jr. also commented on the Redeemer's constant action of chopping his oppenents hands off.
Clockwork took Junior to the Underworld for a mission and took him back afterwards. Apparently, he has little regard for Junior, and Junior in turn doesn't really like Clockwork, because of his inability to save Jeff and Mimi from HIM.