Finn was born in a family of Martin Mertens, a conman, and Minerva Campbell, a doctor. Due to Martin's way of life, he has quite a few enemies, and when confronted by one of them, he had to flee his house with baby Finn in hand. During their escape, an accident occurred, and Finn ended up stranded in the forest.
Luckily, he was found by Jake the Dog's parents, Joshua and Margaret. The pair decided to adopt the baby and raise him as their own. Finn and Jake ended up forming a lifelong friendship as they grew up, with both of them moving to the Tree Fort after the passing of their parents.
Despite pursuing their own interests initially, the two would eventually team up and go on adventures together.
Finn is a simple-minded boy with a strong moral compass and passion for adventure. He always seeks to help others, do the right thing, and thwart whatever evil he comes across. His impulsive nature often leads him to make rash decisions or fall into flashes of anger when his friends are in trouble.
As an assassin, Finn is an expert when it comes to putting pressure on his opponents. His sword strikes have a considerable range and good hitboxes, but what really makes him special is the unique coin system. When Finn hits opponents, they will drop coins that can be picked up either by him or his teammate. These coins have a variety of useful functions. On the most basic level, they buff his Backpack Attack, but he can also use these coins to buy buffs or throw gems, giving him access to a new attack or mobility option with the side special.
As with most assassins, Finn's major weakness is that he's a glass cannon and takes extra damage when getting hit. This makes any mistakes much more unforgiving, as opponents can quickly rack up damage.
Finn is a kid of average height with blue eyes, blonde hair, and many missing teeth. His usual attire includes a white hat with ear-like protrusions, a blue t-shirt, a green backpack, jean shorts, white socks that he rolls down, and black boots.