Terra costume question

How exactly do I unlock Terra's Lightning costume? I couldn't find the right achievement on the HTML guide

You have to obtain the Silk Dress in the Esper Realm in the World of Ruin (very end game content). Alternatively, the back door method for unlocking all costumes gets it too (A Secret Present quest).

Was already on my way there, lol. Good thing it's close by. Thanks

So, what is this? You can change costumes? This is the brave new world mod? Are there more secrets?

Are there any way to missables? Or a way to know these secrets without spoilers? And if there aren't what are the secrets? Is pure luck to get them?

Thanks for answering

This is another mod, FFVI T-Edition. BNW doesn't have these features