With All The Turtles: Donnie usually is the peace maker between his brothers, never starting fights and always tries to stop them when they get out of hand. His only problems with his brothers are Raph's temper, Mikey's immaturity, Leo copying things he see's off Space heroes, and when they all don't act like the appreciate his intellegence. His brothers often get annoyed with him when he acts like he knows everything. Despite everything, Donnie loves his brothers as much as they do him, and he often tries to help them out, back up his two oldest brothers, Leo and Raph, and protect his little brother Mikey from any sort of harm that comes their way.
Leonardo: Leo and Donnie get along very well, but Leo can get annoyed with Donnie when he acts like a know-it all. Such as the time where Raph hits Donnie for doing so and Leo points out his satisfication. However, Leo values Donnie's intelligence much more than Raph and Mikey do. They rarely ever argue, but in The Gauntlet, Leo was yelling at him for not expecting a very complicated mutagen bomb, got mad at Donnie for taking off to battle the Kraang in Rise of the Turtles and getting annoyed at him for teasing him about Karai. Leo, along with Raph, also didn't seem to think highly of Donnie and Mikey as much as a strong fighter, as seen in Mousers Attack, where he considered it unfair that him and Raph were paired up against their little brothers in a fight which caused tensions between them. He even got in Donnie's way when they were battling the purple dragons and made him and Mikey go after April's phone while they delt with the mousers, and to add insult to injury he and Raph kept labling both younger brother the B-team. Though he soon regrets not getting Donnie nor Mikey's help when him and Raph are being attacked by the mousers. In the end, Leo gives Donnie and Mikey new found respect as fighters when they defeated the mousers and recoils his relationship with Donnie, and Mikey. Donnie really respects Leo as a leader, and see's him much more trustworthy than Raph and Mikey, as Seen in The Pulverizer. They seem to be pretty close as brothers. Leo loves Donnie as much as his other brothers and will do anything to protect him.
Raphael: Raph loves Donnie as a brother, but like the rest of his brothers, gets really annoyed when Donnie starts acting like a know-it-all and starts talking in super-smart techno-babble. Raph likes to make fun of him as a geek. Despite this, Raph doesn't tease Donnie nearly as much as he does Leo and Mikey, though Raph does tease Donnie for his crush on April. Raph, along with Leo, also didn't seem to think highly of Donnie nor Mikey as much as a strong fighter, as seen in "Mouser Attack" where he considered it was unfair that him and Leo were paired up against their little brothers in a fight which caused tensions between them. He and Leo kept getting in Donnie and Mikey's way when they were battling the purple dragons and made him and Mikey go after April's phone while they delt with the mousers. To add insults and injuries, he and Leo kept labling both younger brother the B-team. Though he soon regrets not getting Donnie nor Mikey's help when him and Leo are being attacked by the mousers. In the end, Raph gives Donnie and Mikey new found respect as fighters when they defeated the mousers by dubbing them the A Minus Team, which was fine since it was as good it was gonna get, and recoils his relationship with Donnie, and Mikey. Raph and Donnie are polar opposites when it comes to thinking; Donnie often thinks too much and Raph often doesn't think enough. Donnie is Raph's younger brother, even though Donnie's taller than him. He has complemented Donnie on some of his findings and when he turned Leatherhead's subway car into "The ShellRazor," Donnie can get pretty annoyed with Raph's temper and despises his agression and sarcastic comments. In Slash and Destroy, after Donnie is injured by Slash, Raph takes care of him and refuses to leave him, instead he kindly ties his spraid arm. None the less, the two love eachother and as brothers they try to protect and look out for each other.
Michelangelo: Like with his other brothers, Donnie can get frustrated with Mikey's constant lack of maturity and pranks, but he still loves and cares about him. Mikey likes to tease Donnie a lot, as seen from Rise Of The Turtles, especially when it comes to Donnie's 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 (On the cheek) for making smoke bombs. Donnie gets really annoyed when Mikey takes advantage of his inventions as seen in I Think His Name Is Baxter Stockman. He also doesn't seem to like being paired up with Mikey consiering Mikey can be accident prone and annoying but despite that, Donnie and Mikey work well together and seem pretty close, considering they are the youngest two of the four turtles. Their bond takes a step further as the want to prove themselves to their older brothers that they can handle anything in Mousers Attack! and managed to get their older brothers' respect. Donnie can be protective of Mikey just like Raph and Leo and hates it when an enemy hurts him, as seen in I Think His Name Is Baxter Stockman and New Girl in Town. Donnie is Mikey's older brother.
Master Splinter: Donnie loves Master Splinter, as his father, but he sometimes questions his wisdom as seen in Monkey Brains. Splinter praises Donnie for his intelligence and skills as a ninja, but wishes for him to think a little less in battle. He even hugged his father when he came to his senses as seen in I, Monster. Like with the others, Master Splinter is very protective of Donnie.
April O' Neil: From the moment he saw her, Donnie formed a crush on April. He was the first to react when she and her father were kidnapped by the Kraang. Since then they've been good friends, but Donnie wishes they could be more than friends. As the episodes progress, Donnie tries to woe April into being his girlfriend, as seen in Monkey Brains. He even comments on her beauty in Metalhead. Donnie's very protective of April as seen in "The Gauntlet" and would do anything to keep her safe. He was the most upset when Karai tried to kidnap her, but was relieved when she was safe. Even calling her his sweet princess. April seems to sense Donnie's feelings, but seems to still want to be friends with him. There's a game called "Donnie saves a Princess" where his goal was to save a princess who he hoped was April. By the end of the game, he was happy to see her.
Casey Jones: Donatello did not like Casey at first sight, referring to him as "some punk human kid." The relationship is a stormy one, due not only to both Donnie and Casey having feelings for April, but an unspoken dislike that seems to border on a "jock vs. nerd" war. Donnie does, however, seem to respect Casey being loyal and good in a fight, as Casey is equally willing to acknowledge Donnie's intelligence and technical saavy saving the day on a number of occasions.
Karai: Donnie didn't really know much about Karai until he first met her. Despite that he began to hate her for her arrogance and when she didn't help them out in their battle against Justin. Based on Donnie’s observations of her, he does not like her because of her need to cause trouble. He is wary of Karai's tactics and Leo's feelings for her, but does not trust her at all, not even when she offered to help him and his brothers fight the Kraang. Donnie also was a part of the attempted assassination of the Shredder. Karai once called Donnie a name, "Scrawny". Karai may enjoy making fun of Donnie sometimes. However, he came to appreciate Leo's hope that she might except the fact that Splinter is her father and April's growing relationship with her when she excepted it.
Leatherhead: Donnie has neutral feelings about Leatherhead. While he see's him as a valuable ally, Donnie is also terrified of Leatherhead, mostly because whenever Leatherhead looses control, he will grab Donnie by the face and shakes him like a rag doll. But Donnie accepts Leatherhead's help in areas of battling Kraang and even shared in the depression of his brothers when Leatherhead went back to Dimension X with Traag.
Timothy / Mutagen Man : Upon first seeing the Pulverizer, Donnie, along with his brothers, were impressed until they saw how much he wasn't a good fighter. Donnie and his brothers are really annoyed with the Pulverizer. Donnie decided to teach him a few self-defense moves so he could protect himself. Pulverizer was the reason why the turtles lost the power cell to the kraang, still it doesn't seem like Donnie holds any hard feelings against that, considering part of it was his fault too. He also wasn't willing to continue the trainning they started for...obvious reasons. Still despite everything, Pulverizer still looks up to him and the other turtles. In the Pulverizer returns, Donnie was the most concerned about him, considering he was his student. He didn't like having the Pulverizer being used as a spy to gather information on the Foot, and he was the most upset when the Pulverizer mutated into Mutagen Man. He even promised him that he would find a way to get him back to normal.
Shredder: Donnie and Shredder recently met. He, like his brothers, despise Shredder for the things he's done to their sensei and for trying to kill him and his brothers.
Tiger Claw: Out of all the the villains, Donnie seems to hate Tiger Claw the most. Tiger Claw was Donatello's spiritual foe in Vision Quest, and Donnie was able to defeat him on his own. Just like his brothers, Donnie is to Tiger Claw as what Mikey is to Rahzar, Raph is to Fishface, and Leo is to Shredder.
Bebop: Donnie may have a hatred for Bebop since he teases April like a gentleman more than Donnie does, but in an evil way.