Not worth it for lightning main. If you plan on using other mains in the future and want to stay competitive, sage could give you a really good head start. Keep in mind that the longer the game goes, the cheaper and easier sage will be to obtain, so now you'd end up paying a lot more for an early access. That being said, if that early access could help you dominate pvp field on sage/arena ranking, getting more refines and ninja fragments might turn out in a good way. 10k is really expensive though, currently on english version you can get him for 6k coupons (so considering u can transition ingots into coupons over time via try your luck or convoy, its 2k ingots).

Ask yourself: will i play other mains? Do iI have a team for him? Will getting this ninja affect my pvp in a meaningful way? If I overpay, will I dish out enough off of his value to make up for it?

If the answers are yes to all, go ahead and buy him.