The only way to get torchic in platinum is to have a ruby or sapphire or emerald game migrate one to platinum then catch it at the pal park.
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Sadly you cannot catch Torchic in Platinum because it is a starter pokemon of the Hoenn Region. But you should try asking a friend if they have one, one of my friends was nice enough to breed one for me :) ( He transferred his Blaziken from his Pokemon Sapphire Version to his Pokemon Platinum Version) Hope this helped :)
You'll need to trade for it or migrate it from the older Pokemon GBA games. Trade
ill trade you one my friend code is 1805 9351 6951
Torchic is the best.
43 but you must keep him as torchic and not evolve him until then