Eshter is more or less right.
Valencia was conquered by the king Jaume I in the Middle Ages. Before that it belonged to the Moors. Except an inland area which was people by Aragonese, the rest of the territory of Valencia was peopled by Catalans.
In Valencia they speak Valencian, which is the name the Catalan language has there. Scientific fact: it's the same language, just like Dutch and Flemish is the same language, with some differences in vocabulary and pronunciation. In fact, some dialects in Southern and Western Catalonia, are much more similar to Valencian, than say, the dialects in Barcelona or Girona.
However, some people in the Valencia Region there deny that fact and argue that Valencian is a diffferent language from Catalan. It's as ridiculous as if I said that in the USA they speak a language which is not the English, or if Mexicans would say they don't speak Spanish at all.
Nowadays most people in Valencia feel Valencian and Spanish, independently of the fact that they believe Catalan and Valencian is the same language or not. The people who feel Valencian and Catalan is a minority, but in some respects they are the people who strongly defend their culture and language, as Valencian will disappear in 50 years at this rate.
Of course the whole matter is much more complicated.
I'm Catalan from Barcelona. I support the independence of Catalonia. Valencians will do what they choose to do and what they choose to be. We share the language and partially the culture but of course they have the right to remain in Spain. Some far-right people who state that Valencian is a different language, are so paranoyd that they say the we Catalans want to invade Valencia, when most Catalans couldn't care less about it.
Valencia shares many cultural aspects with Catalonia, but is not a part of Catalonia, and there are very few people supporting the independence, most of people in Valencia want to remain Spanish.
No, yes and no.
No.
Are people from Valencia more into the Spanish culture or would they side with Catalunya?