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Cherrygrove Town

Make your way back to Cherrygrove Town, but make sure that you heal your Pokemon up at the Pokemon Center first! Maybe buy a Potion if you don't feel confident in your combat skills (or if you used your first one up already), because you have a fight waiting for you!

As you try leave Cherrygrove Town, that guy with the red hair that you saw lurking outside of Elm's Lab earlier (you might not have noticed him) will bump into you and then challenge you to a fight with a Pokemon that might look a little familiar to you if you took some time choosing your Pokemon...

Boss Fight
Passing Boy Passing Boy (if you picked Chikorita) $500
Cyndaquil Cyndaquil M Fire Lv. 5
69 EXP
 
Passing Boy Passing Boy (if you picked Cyndaquil) $500
Totodile Totodile M Water Lv. 5
70 EXP
 
Passing Boy Passing Boy (if you picked Totodile) $500
Chikorita Chikorita M Grass Lv. 5
67 EXP
 

What a really cool intro to the fight! He shouldn't be that difficult, but he'll be harder than any of the wild Pokemon you've run into. You should be at least level 6 before taking him on, but level 7 is recommended. If you started with Cyndaquil, try use Smokescreen first, then Leer, then finish him off with Tackle.

After beating him, he'll start to leave, but will then come back and show you his Trainer Card because he's just that awesome. Hope you caught his name on it! (Don't worry, you don't have to watch for anything; it's a joke!)

Now go and heal if you want (not necessary if you hop down the ledges back to New Bark Town) and head back to Prof. Elm's Lab!