The Mystery Member.
In the Ch. 169 flashback of the Battle for Ostia, there is a young girl riding on a broom and a cat-like bat in one scene. The girl on the broom is likely the seventh member. She is guiding Nagi, Eishun, and Al to the place where the Princess of Dusk was being held. The Batte for Ostia was the turning point of the Magic World's civil war.
The seventh member as a tomboy was my assumption of the member's identity. Disguise as a female servant, the seventh member would have been easily able to inflirtrated the enemy camp before the battle.
The enemy would have thought that all of the members of the Crimson Wing were males. They were not aware of one of the members being a young female mage because she disguise herself as young male mage. Only the members of the Crimson have known that one of them was a tomboy. She does look like a boy.
The seventh member's dimples have help me to convince that he was a tomboy. Kaolla Sue of Love Hina, and Chao Lingsten has dimples on their cheeks, too.
While having a female member as one of the Crimson Wing was helpful, the female's presence may have cause a lot of tension between two of the other members. Those two members seem to be Jacob Racan, and Negi. While Jack consider to be Negi's rival for other reasons, he may have included fighting for the attention of the seventh member.
She is likely to be just a mage soldier... since it was never disclosed who that was, if she were a member we would have her in the many pictures of ala rubra and probably her name included.
Those pictures contained Nagi, Takamichi, Gatou, UNKNOWN BOY, Alberio, Racan and Eishun. These were the members are shown in all the pictures of the official ala rubra. It is highly unlikely a girl was part of their party, only Asuna who was not really a member of ala rubra, but just some1 they rescued along the way.
The only unknown member is a boy. He is not Fate but has white hair.
okay... since when did our mystery member turn into a girl? : \
and oh yeah, if you're talking about the "girl" in Chapter 169...
check the hair dude... the color of the hair is waaaaaaay different from that mystery member... as different as night and day... literally... :/
Imma try an img tag and see if it fits...
EDIT : too big... D:
Maybe that was just a mage fighting in the war, that has no significance to the story whatsoever?
I will like to point to the dimples on the cheeks of the mystery member. Since Chao and Love Hina's Kaolla Sue had dimples on their cheeks, I have concluded that the Mystery Member was a girl. Oddly, the Mystery Member does look like a boy. The Crimson Wing have known where the Princess of the Dusk(Asuna) was. One of them had to inflirtate the enemy's camp. The logical choice was the mystery member. In the picture of the Crimson Wing in Ch.129, the mystery member is in front of Nagi. You will notice a spot on his left cheek. That spot was a dimple.
At the present time, we still don't know who the mystery member is. My conclusion about the mystery member being a girl was a result of him having dimples on his cheeks, and the fact that some manga comic teams have one female member on them.
i think i heard that there was a connection between that person in chapter 169 to the person who led the attacks on negi's village ten years ago because they both had a cat. i don't think it is a seventh member but a common enemy.