Roll20 is system agnostic and as a result there are no games baked in (implemented). You can set up any game you'd like. If there is a particular dice mechanic Roll20 does not automate you can make a suggestion regarding the dice mechanic or you can use the API (a Mentor level feature) to create the rolling method, or finally, you can simply roll it as you would at home (manually, without the automation). As for character sheets, they are made by the community. If you wish to make a character sheet for a particular game you can request help in the Character Sheet forum and someone should be able to help you.
