1. A good example would be Bill in TLOU. Can't think of any bad ones off the top of my head. I've heard bad things about a gay character in one of the Dragon Age games, but I've never actually played them myself so can't really comment.
2. I don't think there's a "right" amount for any kind of character. If they fit in the story of the game then they should be present, if they don't then they shouldn't. Also, for games with multiple romance options... more options is always a good thing!
3. Yes. I haven't seen much obnoxious stereotyping, so that's a good thing. They only stereotypically gay character I can think of is Zhang He from Dynasty Warriors. Technically his sexuality isn't actually mentioned in the games so he may not even be an LGBT character, but he still stands out for being a stereotype.
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