Thesis: The Appraiser Guillaume is far more than he appears to be - in fact he is superhuman.

Evidence:

1) Invincibility
When you fight Guillaume with Melville you cannot win - it is simply not possible. No matter what tricks you use - even if Melville is at a very high level and has maxed skills Guillaume is just invincible! This alone could be proof enough of the fact that Guillaume is superhuman.

You now might say that Guillaume was rather easily beatable in the other duels - but that's just because he feigned defeat. There is no other possible explanation!

2) Guillaume is the only Support Character with fighting skills
Apparently he was originally planned to be a fighting party member, but he as taken out because he was just to powerful. And to adjust his stats in favour of the gameplay balance would have meant to endanger the tenability of the whole setting because of Guillaume's great 'behind-the-scenes' importance in the Suikoden World.

3) Guillaume' perception is totally different from humanity's
When you meet Guillaume in front of the Cyndar Ruins he pokes the doors with his trident (this alone might be one more indication because the whole act is beyond human understanding). Now, if this scene occurs after the door was opened by the Destroyers, he is nontheless there and he pokes the air! This shows clearly and without ambiguity that his senses work differently - on a higher layer, probably. Mayhap he sometimes sees the past, or he feels the remains of magical barriers he wants to tear down. How could we know what he sees? We're merely human.

4) Guillaume's feud with Koroku
In this article of the reporter Arthur we learn of Guillaume's feud with Koroku "the dog" (yeah, sure).

Guillaume throws Koroku in the fountain Koroku barking loudly
---------------------------------------
Guillaume was witnessed by a resident
swinging Koroku by the tail and throwing
him into the fountain, while he let out
a peculiar cry. After an investigation
it turned out that Guillaume was boiling
with rage because Koroku had eaten his
lunch. 'It was a deliberate action at an
hour when there were very few people,'
said a boy detective. 'Guillaume
has always been antagonistic toward
Koroku. This incident is only the
beginning.' Thankfully, Koroku was not
injured. Is there going to be a feud
between Guillaume and Koroku?

There are many hints throughout the game that Koroku is no normal dog (but this is not the place to discuss this). Obviously Guillaume can see Koroku for what he really is, and therefore does not fall for the perfidious creature's cuteness-trick.

5) Guillaume secretly supports the Flame Champion's Successor
Although this does not directly hints towards Guillaume's superhuman nature, it clearly shows that he is more than just a thief and appraiser.

The fact can be found in the comment box, in which Guillaume writes: [...] I'll support you secretly.

Conclusion:
Based on this overwhelming evidence, noone could possibly contradict me. Many people hate Guillaume because he is supposedly a child molester, but as we learn from Kidd it is just that he likes to play with little girls. Just play.

Guillaume is heavily misunderstood and libeled, and his divine existence is so seldomly realized.

Hopefully this speculation will produce relief.