During my first play through of RSE, I caught a Minun as my third Pokemon. It became pretty clear around the fifth gym that it was weak, outclassed and wouldn't evolve, but I grew attached to that little guy.
Ten years later and it's level 73 and sitting in my PC of my copy of Black. :')
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I've trained Mawile to level 100, twice, and used him once in a competition (got first runner up; he OHKOd a Deoxys thanks to a perfect attack stat). I just love that Pokemon.
The first time I played Ruby I trained the Wynaut you receive in the egg to level 40 without evolving him. Still have him!
I've also taken an unevolved Pikachu all the way through the game in Fire Red, and at the moment I've got an Eevee on my Black 2 game that I don't plan on evolving until after the E4. With the Eviolite he does surprisingly well.
I'm not a GRRL gamer; I'm just another kid who had a Nintendo and never grew out of it.