Well every character has a celestial weapon, at first the weapon is like amazingly baaad, until u put the 2 power up items on it, like Tidus's celestial weapon with 2 power ups:
name: Caladbolg
power ups: Sun crest and Sun Sigil
Skills: Break damage limit, Triple overdrive, evade & counter, magic counter.
but 2 get the weapon and power ups are kinda hard sumtimes, to get Yuna's is easiest
ahh well for any questions just take a look here
FF X Faq page