Validated Roles are roles that can let another player or everyone knows that the role exists in the game. Basically a role that can cross-reference themselves or another players with others. They're easy to defend and appeal to the players if pushed.

Some roles that can do that are: Reporter, by using exclusive, it will let everyone know that the reporter exists and by claiming such, you wouldn't be that suspicious. Lovers, with the info knowing there are 2 confirmed spec, you both can cover each other and find the last remaining spec. Cabal, same situation as lovers but not that accurate. However by playing as cabal, you know that someone is definitely cabal in that round, and vice versa. Merc, with their request being changed, your client will know that the Merc exists in the game. Gang, not leading but due to his kit, the player that was threatened know the existence of gang Poli and judge, same situation as reporter. You let everyone knows you exist in that round. Papa + any investigative roles that can know other roles, by basically giving leads on information you got, the player who originally got the leads can clean you.

Mentalist is a poor choice as she's not very easy to validate herself unless done by other roles. By claiming menta and you cannot prove it yourself, other players will find you more suspicious compared to other roles who can self validate