Trebol was Doflamingo's chief of staff
Exactly. He wasn't stronger than Vergo or Pika and possibly not even Diamante, but he was most responsible for how Doffy turned out to be, his closest advisor, was Sugar's body guard, fought with Doffy against Luffy and Law, the last one to fall before Doffy, and was defeated by Law (the deuteragonist of Dressrosa).
A chief of staff can be among the strongest (Sabo), but that's not a necessary condition. What's important is his close relationship with the captain, loyalty, being his advisor, and his word being respected by the crew.
When BB's men were introduced chilling in Jaya, he was introduced sneaking into Mariejois to suggest his appointment as a warlord. He was the one who hypnotized the guard to facilitate the Impel down break out. He was shown to be extremely dedicated to BB threatening Avalo who suggested becoming captain, and is shown here to be his close advisor. Lafitte is that man for BB, but that doesn't mean he is necessarily strong in a powerscaling sense.