Ramuh is one of many summon spells in the Final Fantasy series. He always has the lighting element associated with him. To my knowledge he has never been a playable character and only a support role as a being to summon during combat and/or an item that grants magic spells. Ramuh always appears as on elderly man with a white beard and a staff of some sort. It's in Final Fantasy XI where he makes his most drastic appearance change.
In the form of an item named Call Magic, Ramuh can be used once during battle (and must be bought again). His effect is chosen randomly from three lighting-based attacks. Other than that he has no additional uses.
Ramuh now become a summon magic spell instead of a one use item. Also changed was Ramuh's number of attacks at his disposal from three to one. Like in FFIII, Ramuh was nothing more than an attack spell.
All of the summons begin to grant other benefits other than an attack or defensive spell. Ramuh also has his first speaking role and informs the party about Espers. Afterwards he willing commits suicide to turn into Magicite and further help the party in their quest. He retains his lighting trademark.
Sticking with the spell-only formula, Ramuh can now only be learned or cast by someone of the job class Summoner. I wont bore you by mentioning that Ramuh is still lightning based.
Ramuh makes no appearance in FFVIII and goes straight to FFIX. Returning to his roots, Ramuh becomes just a summon magic spell and no longer an item that grants magic or stat boosts. You guessed it: he still uses lighting.
