I think the friend is one of the reasons Umi can't/won't express herself - I'd be worried about saying or doing anything to displease her, given how violently she reacts to things. If only Truck-kun could help a sister out...
Yeah that friend is really annoying. I hate annoying characters like that who're just there to impede the progress of any relationship. It's not funny for a "friend" to be that overprotective and controlling; friend really doesn't mean parent.
I actually think the friend is pretty okay. I don't see them as controlling like people are, we've already seen that Umi is very naive and people try to hurt her. I don't think it's weird for someone to be protective of their friends. Also, they literally apologized to paperbag-kun when they realized they were in the wrong.