"Booster Characters" are characters which were made during the this year's Booster event which had players use a website to register for the event and then after registering, a new character with a few special features was added to their game account. The added character was a given a randomly generated name (and a free name reset) and a series of equipment and consumables which unlocked every 15 base levels to help you level faster. Also during the event period all characters made with the website were able to use the Mesmerist as many times as you wanted.
FYI as you have used your Mesmerist reset already if you need another Skill reset you'll want to use a Neuralizer from the Cash Shop or bought from other players to reset those. And if you need to reset your Stat points you can use some Reset stones (sold in the Cash Shop, exchanged from Eden Merit Badges, or bought from other players) and turn in the appropriate amount of stones to a NPC in Payon to return all your Stats back to 1.
Oof, I wish I had known about it sooner, I would've come back and enjoy those bonuses
Btw, I read it was only once when I talked to the NPC, but well, I was sleep deprived at the moment and decided to reset my Assassin to make a crit build (he's not rebirthed yet) instead of a Venom Splasher build (because, for some reason, in my head I thought that build was weird for Assassin, but then saw some people using it afterwards and it was actually pretty decent and not bad e.e)
Btw, could I have used the mesmerist even if I had rebirthed and was 3rd job? Or is it only for non-rebirthed/non-3rd job characters?
And besides, since I can't use the mesmerist anymore on that character, the only way to reset skill set is using the Neutralizer from the shop and if I wanted to reset stats points I'd need to use the reset stones, there isn't a way to reset both of them at the same time just like the mesmerist? Only one at a time by different means?