Pokemon: Super Mystery Dungeon uses a quiz to determine your starter. The answers you give directly impact which starting Pokemon the game assigns you. What answers map to which Pokemon? How can I get a riolu?
Pokemon Super Mystery Dungeon – How to Pick a Starter
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Best Answer
First off, Super Mystery Dungeon lets you just pick any Pokemon you want. At the end of the quiz it will tell you which Pokemon it selected for you, but you can just say no, and pick from any of the 20 starting Pokemon you want. If you just want to start with Riolu, that's the easiest way.
If you really want to go through the personality quiz, then (according to this Super Mystery Dungeon Quiz Guide), the two questions that are relevant in picking your 2 starting Pokemon (player and partner) are the sixth and seventh question.
For the sixth question, the game picks one of five possible questions, giving you four options for which Pokemon you want to be:
A wizard appears before you! He says he'll transform you into anything! What would you want to be?
All right! It's snack time! And today we've got...cake! Woo-hoo! What kind of cake do you pick?
Spring, summer, fall, or winter... Which season do you like best?
We're going to the beach! Best. Day. Ever. Time to kick back and enjoy the breeze. What view do you find yourself staring at?
You've been given a piece of blank drawing paper! You're asked to draw a picture of the ocean. What color do you make it?
The seventh question is always the same question, about music, but the game selects four out of 20 possible answers, each one selecting a different partner Pokemon:
If you specifically want Riolu, you have two options: first, if you get the question 6 that starts "A wizard appears before you!", pick "A wild beast that is one with nature." If not, on question 7, pick the answer that says "Gotta get moving with some EDM" to get Riolu as a partner. If you don't get either of these options, again, you can always just reject the choice the game makes for you and pick Riolu directly at the end.