I can just imagine the episode where Ash's Dartrix evolves.
Some big mean bully Pokemon are picking on some innocent baby Pokemon. Dartrix joins in on the bullying, until one of the big bad Pokemon makes an Igglybuff cry. At that point, Dartrix realizes he's been a big jerk this entire time. He tries standing up to the bullies, but they're too big and just swat him out of the way. With his sense of justice rising, he evolves. He then stalks the bullies to their hideout though the trees, which is packed with stolen food. He defeats the bullies, at night. The next day, the shot starts with the baby Pokemon waking up, and being shocked there's a big pile of food right next to them. The owl is up in the tree tops, watching them. He then returns to the group, which have no idea where he's been since he took off the other day after he evolved.
The rest of the series, he acts as a protector to the group