Pokemon Brilliant Diamond and Shining Pearl are full of side quests for you to complete in exchange for a bunch of neat rewards. You can find them all over Sinnoh and completing them is something you can do at your leisure.
One of these takes you to the Valor Lakefront, just to the south of Veilstone City and to the east of Pastoria City. It's a very short side quest that involves just a little bit of searching around in the grass to find a precious item.
How To Find The Suite Key
This quest is started by talking a lady wearing pink and purple outside the door to her villa. She will tell you that she is locked outside and that she dropped her Suite Key somewhere near the reception area. You'll notice that this area is some sort of resort.
The reception area is within a building that you must pass through to get to Pastoria City (when coming from the north). The Suite Key, however, is not inside the reception area.
You can find the Suite Key close to the northern entrance to the reception area. It is an invisible item so search with the 'A' Button until you find it, use the Dowsing Machine Poketch app, or use the image above as a reference.
Once you have the Key, return it to the lady who lost it. Follow her inside the villa and speak to her again to get your reward: a Lava Cookie. This is a neat item that can be used to heal a Pokemon of its status effects.
Don't worry, MultiVersus is more Ultimate than Brawl.