Hi Tony0075,
maybe you can try this step to reach that :
1. the prerequisite is you have basic programming skill from HwitVlf's PSX Hacking Info 1.1 and tools for debugging such as psx debugger 1.13
2. open Brigandine and try make save state just before you got Merman or Tritons quest in Organize Mode so we can use this for debugging later
3. after you got quest that give you Merman, try to find merman id 3E or its strength for example
4. after you got either that new Merman id / strength, then use break feature from psx debugger to watch read/write address from Merman id for example
5. our last state is in organize Mode, let's try execute it by press Start button then choose Execute in-game
6. the game will stop at quest that give you merman. THERE !! you start from here, try identify merman id 3E from that disassembly. which memory that handle and register merman. I hope you can find it. Good luck !
btw...
I am sorry I can't help you more than this because my research not touching any specific quest related code
