Serpent

Roa's nickname.
The Dead Apostles gave it to him, in reference to the way he lives on through reincarnation. Given that he dabbled in alchemy, it's quite fitting.

Tsukihime Data Collection: Tsukihime Dictionary
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Roa's nickname. The Dead Apostles gave it to him, in reference to the way he lives on through reincarnation. Given that he dabbled in alchemy, it's quite fitting.
Of course, when you hear alchemy the first thing you think of is Sion, but Sion is an Atlas alchemist, which is something totally different from those alchemists that devoted themselves to Western magecraft. It seems that the Atlas alchemists don't think much of Roa for that very reason, even though they both subscribe to the common idea that “everything flows.” Also, contrary to what you may expect, Sion too is possessed of this same elitism, and looks down on Roa on principle.
…Huh? Then are Sion and Ciel actually on bad terms…?

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised