Some guides required here!

I want Headdy (locke) to stay ability-less until he finds the object that "changes Slot for GP Rain", that once equipped, lets it gain access to the ability, GP Rain that is. Is it possible to do such thing?

Yes it's possible with some ASM coding.You could set a branch in the appropriate place in bank C3 and write the ability in the SRAM when it's character 01 and when it's the good relic. The Abilities range from $1616,X to $1619,X in the SRAM.