How to become a Brawler

Look for Kyrin in Nautilus navigation Room to accept job advancement quest "How to become a Brawler" quest before talking to Kyrin again

Ask for more information on Pirates under "MISC." and Kyrin will transport you into Pirate Test Site Pirate's Deck

Kill 15 OctoPirates to get 15 Crystal of Strong Power

After getting all 15 Crystal of Strong Power, talk to Kyrin again at the left of Pirate Test Site Pirate's Deck to be transported back to Nautilus navigation Room

Talk to Kyrin to complete the "How to become a Brawler" quest

After that, talk to Kyrin again and she will make you an Infighter, provided you have used up all your 1st job skill points