For sidekicks, there are a few simple questions you must ask:
1. Can they die of their own idiocy?
2. Do they get in your way?
3. Would the game be any different if they weren't there? (Such as, you can't leave the area until they're within a certain distance from the door, or they're necessary to hit a switch)
4. Can they actually kill things?
5. Do they speak outside of cutscenes/scripted moments?
If the answer to any of these questions is "yes", it's not a good sidekick. If it's yes to all, the people in charge of writing their programming, scripting their events, and the voice actor must suffer horrible deaths involving knives, fire, vinegar, and a very hungry squirrel with laser eyes.
If they have none of these, then it's all good. RE4's Ashley came close. She stuck right behind you, would only die if you sucked at the game, and when you told her to wait, she stayed still, and didn't get bored after ten seconds and decide to chase a butterfly. Unfortunately, she took ages to catch you up if you left her somewhere, and her voice was squeaky as hell. Kotor's, on the other hand, were great. They didn't speak, did virtually nothing in combat unless you told them to, and were interesting characters that you could really get to know.