Donatello (Teenage Mutant Ninja Turtles) - Wikipedia
It may be because during all the four seasons the writers have prefered to develop more the relationship of Donnie-April instead of Casey-April. Donatello is happier than he ever thought he would be with April as his girlfriend, but when new milestones in their relationship are approached. "Nick would never pair April and Donnie up, parents would never be okay It seems like they could favor this kind of relationship, but unlike.
April O'Neil hummed as she ran the paintbrush up her big toe. She had picked a purple polish. Not her usual color, but it had been a whole week since she had seen her boyfriend Donatello and she was feeling a bit sentimental. After the turtles' latest run-in with Dogpound the Shellraiser had been completely obliterated, leaving Donnie to build the whole thing again from scratch as soon as he could so the turtles could stay on top of tracking the Kraang who Donnie had been particularly determined to thwart ever since he discovered they were after his April.
Girlishly, a giggle bubbled up from April's throat. She was his April now. Ever since that month-ago awkward confession and first kiss, she was his. And he was hers.
Which brought her right back to being frustrated that she hadn't seen him in so long. She did have a big school project to work on, so she supposed it was for the best that she hadn't been distracted- but just texts from the scientific turtle hadn't been quite enough for her. She was ready to have Donatello back in her arms. Speak of the devil, a tiny knock came from her window at just that moment.
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. 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 at all. Donnie is Raph's younger brother, even though Donnie's taller than him. Donnie can get pretty annoyed with Raph's temper and despises his agression and sarcastic comments.
In Slash and Destroyafter Donnie is injured by Slash, Raph takes care of him and refuses to leave him, instead he kindly ties his sprained arm.
Nonetheless, the two love each other and as brothers they try to protect and look out for each other. He also doesn't seem to like being paired up with Mikey considering Mikey can be accident-prone and annoying, but despite that, Donnie and Mikey work well together and are pretty close, considering they are the youngest two of the four turtles.
Donnie is Mikey's older brother.
Jeremy Howard on Donatello's Relationship with April O'Neil, "TMNT 2" Expectations
Donnie loves Master Splinter as his father and teacher, but he sometimes questions his wisdom as seen in Monkey Brains. 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. Donnie didn't really know much about Karai until he first met her. Despite that he began to hate her for her arrogance and the time when she didn't help them out in their battle against Justin.
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.
Out of all the Turtles, Donnie is the one April is closest with. He has tried numerous times to get her attention, but April is either indifferent or oblivious to his advances.
She does, however, seem to care about him as a friend. April kissed Donnie on the cheek in Target: However, despite this, April probably does not have a crush on Donatello in the same way he does her, consistently showing more interest in Casey however infuriating she may find him and being polite-but-clearly-a-little-uncomfortable when Donatello calls her for example "my sweet princess" or gives her gifts such as the music box in A Foot Too Big.
Aside from Donatello, Leo appears to be the most protective of April in the group and is supportive of her efforts to discover information that will lead them to find her father. Leo, aside from Donnie and Splinter, has been the most to interact with April. As of yet, Raph and April have not interacted much onscreen. He does, however, seem to show deference to her when she catches him and his brothers teasing Donatello for sustaining injuries from a lab monkey or when he laughed at her for being chased by Pete, a giant Pigeon.
However, they do remain good friends, and Raph shown signs that he cares about her at least as much as Leonardo and Michelangelo do as seen in Panic in the Sewers where he agreed with the others that she shouldn't go spying on the Foot, and in Karai's Vendetta when she spilled Kraang water on herself.
April O'Neil (Character Interactions)
He is also the one that goes to rescue her in The Kraang Conspiracythough both he and April make satirical comments about it. Most notably are the handful of occasions when Raphael refers to April as "sister" when making a sarcastic comment, and one point in Race with the Demon!
Raph and April do seem to get along most of the time, probably because both have very feisty, active personalities.
- Donatello (Character Interactions)
Whilst Donatello clearly has a crush on April, Raphael is more like a big brother to her. Mikey and April have not interacted much onscreen. Although she is his first human friend, Mikey says she doesn't really count because she is obligated to like him since he helped to save her life. Despite all that, they remain good friends and Mikey Cares about her. Examples are shown in The Gauntlet Where he offered to make April some soup or get her a tissue when she was upset over the fact Pete was after her.