Lesina was robbed from the cradle at birth during the Hylian-Gerudo war and then made to serve in the Hylian Army when she came of age. She fought small battles with her Troop under the command of Commander Gerald (who will be mentioned eventually) and became the second-in-command of that Troop. It's not really an important title that would make her famous, but when the war ended she was largely sent back home to fulfill whatever duties the Kingdom is asking of her at the moment. There's most likely more than one gate to watch, right? I didn't specify that she was guarding the literal castle gate, it could have just been Green District.

Sibela likely could have met Lesina during her Squire training if they were in the same Barracks as young girls. If you want Sibela to know about Lesina, that wouldn't be unlikely. [@Inkarnate]