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You can evolve Riolu with the Sun Ribbon.
Some missions can give a reward. It can be a lunar ribbon.
you get it by completing the special "today's oh my gosh"
If you have Pokemon mystery dungeon explorers of sky after you complete the third special episode your rewarded a sun ribbon
In Explorers of Darkness you can get it as an occasional reward for completing a job. And in Explorers of Time, you can't get it, and have to get wondermail to get this reward.
you need a sun ribbon and its IQ needs to be 1 star
find it in a dungeon
You use a lunar ribbon and you can only get a lunar ribbon in explorers of darkness and a sun ribbon (eevee evolves into espeon) in explorers of time
you have to have a razor fang and a lunar ribbon because normally they evolve with the razor fang at night
you have to find a sun ribbon which is only available in explorers of time.
get a sun ribbon and train for a while
bring him to luminous spring when his IQ is past 1 star and when you have a sun ribbon in your bag