From the first TMNT movie credits song (emphasis mine):
Now our ace reporter was hot on the trail
Determined to put these crooks in jail
She spied the bad guys and saw what happened
But before she knew it, she fell in a trap and got caught!
Yeah, she was all alone
With no friends, and no phone
Now, while this can be read that she didn't have any friends with her in this moment to help her, after thinking about it, I can't recall April having any human friends other than Casey in any series. While there are a few other human characters scattered throughout the shows--kids, co-workers--April doesn't seem to be actual friends with any of them.
There is this list that gives examples of April's friends, but the vast majority are either mutants or super-human/super-natural, and the ones that aren't seem to be inconsequential, ie her roommate Trish.
Why does April have so few human friends? Is this part of her character ever explained?