Friends and Family
With All The Turtles: Mikey loves and adores his brothers; he loves to prank them, crack jokes about them, and even bug them; the typical little brother stuff. His brothers get annoyed when he acts immature, and Mikey hates it when Raph can't control his temper, Donnie acting like a Know-It-All, and Leo when he acts bossy. Despite everything, His brothers love him and are very protective of Mikey considering he's the baby of the family. Mikey in turn is very loyal to them and is willing to do everything he can to help them and back them up. He's very close to Raph and Donnie and looks up to Leo like a parental figure.
Leonardo: Mikey acts like a baby brother mostly with Leo, as he is Mikey's oldest brother and Leo acts like the oldest big brother mostly with Mikey, such as being overprotective and not getting very annoyed with Mikey's antics unlike the other two. Mikey really cares about Leo and shows it, like in New Girl In Town where Don and Raph ignore Leo meanwhile Mikey hugged Leo and waved at him as he left the room on Raph's orders. They are opposites when it comes to being serious, as Leo can be very strict and serious while Mikey is very immature. Although they don't seem to be close personally, they care very deeply for each other and it is seen in a few episodes, such as when Leo and the others rescue Mikey in New Friend, Old Enemy Leo pats Mikey on the head and smiles in relief that his youngest brother's alright. Mikey was mad at Leo, and Raph, for treating him and Donnie like they weren't strong fighters in "Mousers Attack!" but managed to reconcile their relationship in the end. Mikey tends to see Leo as a parental figure at times, always going to him for answers, and he even manages to almost get Leo to give into his puppy dog eyes. In "Attack of the Mega Shredder", Leo went back to save Mikey after he had been eaten in an attempt to slash it open but fails and nearly got eaten himself, he looked at Mikey's Grappling Hook and gave a sad look believing he was deceased.
Raphael: It is shown that Raph gets annoyed by Mikey the most out of all his brothers, and despite that, Mikey teases him a lot. It's whether they're bickering, or they're on the crazy side of the team together. Raph, along with Leo, showed Mikey, and Donnie, very little respect as a fighter in "Mouser Attack" labeling them as the B-Team, until the end where they prove their worth and Raph showed his respect for them by calling them the A Minus Team, which was fine since it was as good as it was gonna get. Despite everything, Raph really loves Mikey as seen in New Girl In Town, Raph actually hugged Mikey and mentioned how he was afraid of losing him, thus showing how much he loves him. Another example is in Cockroach Terminator, when Mikey was kidnapped by Chong, as bait, Raph decided to put his fears aside to go save Mikey. He was also really concerned when Mikey was in danger in "New Friend Old Enemy." In that same episode, Raph managed to comfort Mikey, who was Sad that Bradford was using him, stating he was awesome and anyone would love to be his friend. Raph seems to be very protective of Mikey, as he, along with Donnie and Leo, threatened Leatherhead to stay away from him in It Came From The Depths. Mikey was also concerned about Raph in "The Pulverizer" when he was poisoned by Fishface. Raph even cares for Mikey when he saves Mikey from drowning with the Squirrelanoids and protects Mikey from getting killed by Slash. Especially in The Invasion, Raph and Mikey really bring out their love and compassion for each other. Mikey and Raph seem to be quite close, although they fight a lot.
Donatello: Mikey likes to tease Donnie a lot, as seen from Rise Of The Turtles, especially for his crush on April. yet Donnie seems to criticize his idiocy the most out of all his brothers (see I Think His Name Is Baxter Stockman). Mikey seems to like Donnie's inventions the most, and loves to test them, as seen in The Gauntlet, where Mikey kisses Donnie brotherly (On the cheek) for making smoke bombs. They still love each other as brothers, since Donnie is Mikey's older brother. Their bond takes a step further as the want to prove themselves to their older brothers that they can handle anything in Mousers Attack! Donnie is as protective of Mikey as much as Raph and Leo are, even going along with the threat of attacking Leatherhead if he harmed Mikey. Mikey also loves testing out Donnie's inventions, and the two work really well together as a team.
Master Splinter: Michelangelo is loyal and obeys Master Splinter's rules. However, Master Splinter is also highly aware of Mikey's lack of maturity. When Splinter was asked if he had ever considered naming Mikey the leader of the team he replied, "No. That would have been....wrong." But he still loves Mikey, as much as he does with the rest of his brothers. Splinter even acknowledges Mikey's wise choices, such as when Mikey rescued Leatherhead from the Kraang, or when Mikey (Accidentally, considering there was no one else to blame) said that Splinter shouldn't have allowed them to go to the surface, because he taught the turtles to fight as individuals, not as a team. Mikey even gave Splinter a hug when he came back to his senses in "I, Monster."
Karai: Mikey at first didn't know her. But when she betrayed the team, when Raph asked them if they liked her, he replied "I don't know her, but now I ALREADY HATE HER!". When Splinter told the turtles that Karai was his daughter, Mikey, completely taken aback, yells "Karai's our sister?!". Since Karai finally believes the truth and joined the good side, Mikey began accept her into his family. In Serpent Hunt, he tried to remind Karai to be herself by tell her to remember him as her brother.
April O'Neil: Mikey loves making friends (especially with humans) and really likes having April as a friend, even though he believes that she is only obligated to like him since he helped to save her life. Though, despite that, the two seem to be very good friends, even offering to get her a tissue and make her some soup when she was upset when she was stalked by a giant Pigeon. He was also shocked that Donnie suggested they use her as bait.
Leatherhead: Mikey was the first out of all his brothers to be willing to rescue Leatherhead from the Kraang. Despite his brothers' fears and warning that it wouldn't be a good idea, considering the mutant alligator had a temper (which was no real surprise), Mikey managed to convince them to bring him back to the lair. While the other turtles went to look for the Power cell, Mikey took the time to befriend Leatherhead, who really opened up to him. Mikey went after Leatherhead when he went on a rampage and attacked his family when Donnie showed the Power cell to Leatherhead. When the Kraang attacked, Leatherhead handed the Power cell to Mikey and his brothers and fought the Kraang so they could escape. Because since the youngest turtle trusted him, he decided to trust him. Mikey is the only one of the Turtles who can calm down Leatherhead when he goes berserk. Leatherhead participated in helping the Turtles in TCRI, where he sacrificed himself so the turtles could escape. Mikey was the saddest of them all to see his friend go. In Into Dimension X! Mikey managed to rescue Leatherhead from the Kraang.
Mondo Gecko: At first Mikey was disliked by Mondo when he gave the skateboarder a lame name, but earned his trust him after saving his life. When Mikey was kidnapped Mondo apologized and explained that it was a ruse set up by Fishface telling him that Mikey and the others are his enemies. But after seeing through Fishface's lies he rescues them and he competes through his race. Mondo thinks Booyakasha is a lame catchphrase whereas Mikey thinks Cowabunga is more lame, though they grew to like both later on.
Renet: In the events of "Turtles in Time", Mikey began to develop a crush on Renet, the time travel mistress apprentice. While the others tend to dislike Renet from bringing them to the Middle Ages and being a bit clumsy, Mikey instead admires Renet for her appearance and her cool fighting style. Mikey even referenced having a date to riding a Middle Age horse together. It is even seen that Mikey cares for Renet, as he was chasing after her during the final battle with Savanti. In fact, (unlike Donnie) Mikey gets his first kiss from Renet at the end of the episode when he asks her out. Renet in reply smiles, saying it would be nice to do that next time and proceeds to kiss him on the cheek.
Shredder: Mikey and Shredder have met and, along with his brothers, is now at war with Shredder and his Foot Clan. Mikey hates Shredder for what he's done to his father, Splinter, trying to kill him and his Brothers, and for all his misdeeds.
Rahzar: Mikey tried to befriend Bradford on an online social network. Bradford, who was working for Shredder, decided to take advantage of the situation and pretended to be Mikey's friend to earn his trust, even teaching him a move called "Death Dragon." When Mikey learned the truth, he was heartbroken, but got his revenge when he and his brothers lured Bradford and his accomplice Xever Montes, into a trap, used the move he taught him against him, and un-friend him from the social network. He even called Bradford his "Ex-friend" and assisted his brothers in kidnapping him, in hopes of rescuing Mr. Murakami in Never Say Xever. Bradford considers Mikey a freak as much as he is an idiot, but they managed to work well together in Baxter's Gambit.
Fishface: Mikey and Fishface don't interact much, but when the turtle robbed him of his errand boy (Mondo Gecko) after their race, Fishface's hatred for him increases. Also he hates the name Mikey gave him and calls it an insult.