What stood out most in Kuroda's mind about the Kekkaishi were his eyes, which were dark and terrifying. Aside from that, he wore largely traditional clothing for a monk or Kekkaishi.
The Unnamed Kekkaishi was first seen during a flashback of Kaguro's past. Before Kaguro became an Ayakashi, he found him. The Kekkaishi sensed the darkness within Kuroda (Kaguro's pre-Ayakashi name), and proclaimed that the only reason that Kuroda could cut through Kekkai was because he was no longer human, but a demon, and as such, it was a Kekkaishi's job to destroy him. The Kekkaishi then used a Zekkai, but the very sight of it scared Kuroda away. Part of the reason Kuroda eventually became an Ayakashi was so that he would have enough power to defeat the Kekkaishi.
Powers & Abilities
Hazama-Ryu Kekkai technique: During his encounter with Kuroda, he briefly used a Kekkai, then immediately produced a powerful Zekkai that surrounded his entire body (he also shouted the technique's name, which is how Kaguro knows it).