Lawful/Good play off. Make a situation where the 'lawful' choice and the 'good' choice are opposed. Have your protagonist pick one side, and your rival hate him because he supports the other. Works best when you are good and they are lawful.
Another possible twist, aside from neutral-good PC as the trigger for Lawful-neutral/lawful-good going very very bad, is the introduction of a lawful-evil catalyst, or a chaotic good catalyst.
(I haven't played any of the pokemon titles, since they aren't the kind of jrpg I find myself interested in, so I am reading from context.)
Those work to make a lawful good char either have an internal conflict on their alignment (lawful good has to reconcile lawful from evil, to satisfy good-- or sacrifice good to satisfy lawful.), or cause them so much cognitative dissonance that they are then mired in an identity crisis, and easily manipulated, and subsequently driven quite evil via betrayal.
Lawful good is one of the worst combinations of character alignment, since they represent an extreme of both axes. The same is true of chaotic evil, since the extremeness of both sides prevents orderly planning, or the invokation of a zanatos gambit (say, running an orphanage and pulling a Dr Jeckel to conceal Mr Hyde, and employing his vital position in society as a shield: destroying or ousting him would cause even greater harm than good. Lawful evil chars often find ways to make this kind of thing happen, and satisfy the evil part of their alignment via their hidden persona, and the perverse joy of subverting order and goodness for concealing evil purposes and actions.)
The most effective protagonist is a neutral good, and the most effective villian is lawful evil, or lawful neutral. (Lawful neutral villian is a trip to write for, but also difficult. Its too easy to cast them as deluded and evil, rather than neutral. A lawful neutral won't go out of his way to cause harm or misfortune, but won't have any qualms about causing such either. Will blithely demolish a 500 year old church to put in a parking lot, if the church loses property ownership of the lot, and the new owner gives the order. Same for kicking people out of shelters, or taking kids away from loving parents due to trivial legalities. He feels neither guilt nor remorse, just duty and obligation.)
Lawful good antagonist needs to suffer from some form of deception (does evil, beleving he is doing good), suffer from an alignment confict (decision forces him to go against one axis to atisfy the other axis, with consequences), or suffer a radical alignment change from one of those two.
(cecil (darkness knight) from FF2US suffers from deception archetype when he gets used to destroy rydia's village, and his friend the dragoon suffers the latter, while working for golbez, until he can redeem his real alignment.-- just as examples from another jrpg)
Catalyst characters aren't really part of the main plot, just accessories that initiate a change just by being present. They can be somebody that dies because of the antagonist's actions in the past, which initiates the identity spiral, or they could be a sadistic lawyer who gets off on petty suffering by manipulating the law, causing the antagonist to do the dirty work of doing said evil, to satisfy their lawful alignment, etc. The contribute to the conflict simply by existing, and have no real stake in the conflict.
To give implementation advice, i'd need to know more about how you intend to use this character.