Do you mean a gym leader?
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
The water Pokemon gym leader is in Pastoria City.
yes
The eighth gym leader in Pokemon platinum is the one in sunny shore city.
You use your strong(est) Pokemon to kill the Gym Leader's Pokemon.
You can't
volkner
No, it is not possible.
his name is cyrus he is the leader of team galatic
That'd be Volkner who remains the Gym Leader there.
The forth gym leader is in Veilstone City
The 6th Gym Leader in Pokemon Platinum is Byron in Canalave City. He specializes in Steel-type Pokemon and has a Steel based Gym that looks like a construction site.