So, you have a new campaign, and you have a character playing who wants a special familiar. You also need to have some rewards created, but you don’t want to give the party a magic item that they may pass on to another party member when the character dies. So, why not introduce a special reward in a special way? Introduce the player to a “living spell” that looks like what will become his new familiar, and its asking for help. When he touches it, it disappears and a new spell appears in his head. He can now cast a variant version of find familiar that summons a kercpa, and as he levels up, you can level it up as well for a reward.
But the main changes to this spell are that this familiar is more able to help you by attacking, but also, they are less disposable since you must give up hit die each time you summon them and they remember how they were treated in the past. It is also a spell which can be learned by anyone with spell slots of any kind. Also, unlike a sidekick, you can send this little helper away whenever you think you are going somewhere particularly dangerous. Obviously, this same concept can be applied to numerous creatures as well, and I’m more writing this to share the idea. Take the concept and apply it to your own campaign as you like. You may even consider things like having a pair of these creatures who are the spirits of kercpa lovers, or father and son, such that players could have their familiars interacting with each other as well.