From a roleplaying/realistic point of view, which starter would be the best out of those three in your opinion?

I believe bulbasaur has enough tools to deal with both. It can use its vines to shut charmander and keep it from breathing fire. If the game has a manga-heavy reference it can simply point its tail away, effectively making it useless. Squirtle is weaker and cant hurt him by default, unless it somehow obtains an ice type move.

Bulbasaur also extends its usefulness out of battle. It can put a bunch of enemies to sleep, poison food or water, weaken a much stronger enemy by simply planting a seed on it, etc. It can also replace energy like a plant, saving you potions and food. On its final stage it keeps all those features and can be used as transport. It doesnt mind water at all and although he has no access to water type moves, it can simply swing its paws around and swim across a lake or river.

Im a big fan of charmander design/nostalgia/game wise, but other than flying, heating and cooking it provides little outside of battles in the long run. I'll be traveling the world and most pokemon can fight. I'm going for those that can do something else, as I will be relying on them for other things, unlike in the games where you're safe most of the time.

Squirtle fans dont get me wrong, I really like it as much as you, but in comparasion to these two it doesnt stand out so much. Yes it can take out a fire, travel around, provide a strong defense etc etc, but most of those things can be done by the others also.