Stormo is a creation by Princess Bubblegum that was made to combat Goliad for all eternity. Bubblegum made him using Finn's DNA so that's make's Stormo Finn's son.
Stormo though was shot by Trevalyn and dead or so Vilgax's allegiance par Discord though. Professor Pericles recovered the samples needed to make Stormo and then Stormo is recreated by Pericles, Finn, Bubblegum, Bender, Skipper, Heloise, Mandark and Phineas that very day and it's revealed that our unpredictable leader did something impulsive during the creation
It's revealed that Skipper, Bender, Finn, Heloise and Bubblegum went to the Candy Kingdom and retrieved him. With him, Skipper and Bender got a hold on their alley Jake Muller (the son of Albert Wesker) and schemed a perfect plan. Their Plan was to create a check up service for all members and when Heloise and House got to Jake they did all that and with a blood test they gave it to Bender and Skipper who injected Stormo with the virus so they could activate it's full power if it was killed. So when Trevalyn killed hit it returned more powerful than ever but it still held Finn's mindset being made from Finn's DNA.