For this trophy you need to find someone who looks like the Prince from the Katamari games and get him to jump on BOY's back. If you don't know what Prince looks like, just take a look at the trophy picture.
The easiest way to do this with minimal searching is to hold up on L3 and press Start, then choose Planet and The Moon. Once there, go into your house, hold right on L3 and press X twice. This will put you on a new map. Keep doing this and eventually you will be put on a map with about 50 Princes and a dog. Then just stretch out by holding L3 and R3 in opposite directions and the trophy will unlock when a Prince jumps on BOY.
On this side note, this also a great place to get
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