All rats are sweet. All rats have their own personality and while most rats are friendly and outgoing, some can be shy. Females are not fond of holding still, are less likely to urine mark, and are more prone to getting tumors. If you decide to only get one rat, you must spend a lot of time playing with him. It's best to get at least two.
Rats are very social and get lonely by themselves.
If you do decide to get two, be sure that they are the same sex, or have on or both neutered. If you decide to get only one, spend a lot of time playing with it. I have a rat, and I just bought a new one. For several weeks, you need to keep the new rat separate from the old rats. Keep a good watch on the new rat.