Alright Coronis,
If you're looking for a 6th member then take a Snover, and name it WhoAmI?
This Snover is very confused and is sure he is an Empoleon, so he can only use moves knowable by empoleon. He must solo a gym leader of your choice (bar pokemon which chimchar must do) and Mars, Jupiter or Saturn (again, bar pokemon which chimchar must do) to evolve. Once he evolves he can also use any move also known by another pokemon in your party.
and @ Slowpoke Master,
Take a nidoran male. He/she must solo 2 trainers in Lt. Surges gym and at least one trainer in every gym after that as well as one elite 4 member. The moves that can be used depend on the nature:
Adamant, Brave, Naughty or Impish: He learn any TM moves unless they come from a gym leader which he has soloed at least two of their pokemon.
Relaxed, Docile, Quiet, Calm or Gentle: He can't use Thrash, Mega Punch/Kick, Megahorn or any fighting moves apart from counter and seismic toss.-He's too chilled for that
Quirky, Bashful, Naive or Lonely: He can only learn moves by level up, move tutor or HM moves.
Hardy, Modest, Mild, Timid, Lax or Sassy: He can only use moves with 60 or less base power or 5 or less pp
Rash, Hasty or Jolly: Can only use attacks with poison or ground type and hyper beam. He must use hyper beam at least once in every rival battle.
Careful, Serious, Bold: You can choose any of the above.
Lastly, He loves to the sea, so every time you go surfing, you must have him in your party.