Unlike Pokemon though, SMT and Persona encourages you to fusion and swap out demons though. While you could in theory keep using your low level starter demon, it would be very grindy and not worth it. As an example,
here's a link to the fairy class. You'll notice that Pixie, for example, tends to be a low level demon, quickly overshadowed by the others in that class. Think of it like a Caterpie or something, a low strength demon for early game you aren't supposed to actually keep around.
As for fusion, it's the mechanic used to, well... fusion two or more weaker demons into a stronger one. It's kinda like evolution in Pokemon in that sense, just that you get a completely new, stronger demon which is usually of a different "type".
In short, don't get attached to your demons.