You can but only after defeating the red gyrados.you find him in the poke mart thing in mahogany town...
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the guy with the black cape[also known as lance]is the Pokemon league champion after you meet him in mahogany town.
in the Pokemon league
You meet him at the lake of rage after fighting or catchig the red gyrados.
your starter Pokemon
You must be at the Lake of Rage and use Surf to swim towards the Red Gyarados. Either capture or defeat the Gyarados and then make your way back to the bank of the lake, you will now meet up with the black cape guy.