The area circled in red simply does not look like hair. It's something in the background.
Clearly, the man's costume was originally designed for Serge, but that only reflects the artist's original intent. Just as Guile was planned to be Magus, yet simply is not, this unnamed figure might not be Serge.
Honestly, I think the fact that the figure's face is obscured means that we are NOT supposed to know who it is. In other words, there is no "correct" answer, unless or until Kato answers the question for us.
That said, my money is on Magus. As was stated above, the manner of dress seems to jive with Zeal. The design may be based on Indian wedding costume, but since no indication of a marriage is given in the game itself, I think that claiming this is a "wedding picture" is a little ridiculous.
Besides, consider this. Why would Schala be writing about searching for Serge in her diary if she already found him and they got married? No, I really think it's either Magus or some unknown and totally unimportant (to us) person, like a servant, perhaps, in the picture with Schala.
OdF