In the early stages, Camelot had a hard time choosing a 4th party member; Sheba, Felix, and Jenna were already decided on during the development of The first game
Since Alex was at the time, the other most well known Mercury Adept, he was the original choice
However, Camelot found that The Lost Age would be too easy since you already had a full party, and they still wanted to give the game the same feeling as the original game did with the players, the feeling of excitement that new people joined you
Piers was not a last minute edition, Alex was scrapped as a char within less than 6 months, Camelot went ahead and started to create a new character, originally he was of course gonna look like Link since the original Lemurian sprites resembled the Hylians (we would of had at least 1 elf character at the time)
The change of how the Lemurians looked came not too long after, since Camelot felt it was too cliche to add elves, thus they decided to make Piers and the other Lemurians resemble Mia and Alex, but with an "Islandy" feel