The new party member NEVER gets to keep their old super awesome gear. Least in nearly any video game I've ever played. They also come in at some pre-set level with JUST passable for the area you get them gear, in spite of perhaps previously being a boss or having any number of good excuses to be way more awesome than you. Or they get magically neutered down to your level for some reason. Marceline knows something would have happened to rob Peridot of her high-tech junk and so just decided to cut to the chase.
Poor Peri! Probably spent a lot of time making those limb enhancers...
I vaguely recall a couple of games that let the newbie keep some stronger than average equipment.
I also recall whenever that happened, it was instantly switched over to my main person, so it never really mattered much.
Ah yes, Beatrix from Final Fantasy 9 is the best example of this. Although it isn't gear so much as her superhuman powers.
I have seen plenty RPGs that let them be just as powerful, but have bad further stat growths. Like a reverse Est character. Others just let them be the powerhouse as a reward for the big brain intrigue they just pulled. Or have him be Roman from This War Of Mine.