Even though the title subliminally indicates the opposite of what it claims to present, it is not supposed to.
I am as serious as I could possibly be: The friendly Yan is in fact FFIX's strongest enemy, in terms of its stats and in terms of its behavior in battle. Beating the friendly Yan without characters whose stats have been increased to their limits while simultaneously having to resort to both the most useful Support Abilities (Auto-Regen is FFIX's custom infinite HP cheat...) and abilities that deal damage regardless of the enemy's defensive stats (Thievery, Dragon Crest, Frog Drop, Limit Glove, Dark Matter) is most likely the greatest battle challenge FFIX has to offer. Interestingly, the friendly Yan does not only counter all attacks, provided they ever connect, but it also spontaneously assaults one's party if one does not give it a Diamond soon enough.
Incidentally, the friendly Yan is the only friendly creature that does not flee when it is attacked. Moreover, it is the only one which one can defeat as often as one desires to (easily) acquire 50 AP in a single battle and to obtain Rosetta Rings without having to pay for them (at least not in a pecuniary sense).

Tracklist:
1) Hymn of the Fayth (FF X OST)
2) Birth of a God Remix (FF VII OST) http://www.mediafire.com/?cp4961u92fq...
3) Tango of Tears (FF VII OST)
4) Hailfire Peaks (Banjo-Tooie OST)
5) Dancing Mad (FF VI OST) http://www.mediafire.com/?abrcrj9d422...