Castiel also became a Seraph after God resurrected him from the dead when Lucifer killed him in 5.22 Swan Song. This is later confirmed when Castiel refers to himself as a Seraph in 8.05 Blood Brother.
The only other angel to be named as a Seraph was Akobel who was a Seraph of the Six Choir.
Seraph Angels in Lore
A Seraph is a type of angel mentioned in both the Hebrew Bible and in the Christian ranks of angels.
...I saw the Lord sitting upon a throne, high and lifted up; and His train filled the Hekhal (sanctuary). Above Him stood the Seraphim; each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew.
– Isaiah (6: 1-3),
The Seraphim make their first Christian appearance in the Book of Revelation iv. 6-8, where they are forever in God's presence and praising Him constantly: "Day and night they never stop saying: 'Holy, holy, holy is the Lord God Almighty, who was, and is, and is to come.'" The Seraphim and the Cherubim are, in Christian theology, two separate types of angels. The descriptions of the Seraphim, Cherubim and Ophanim are often similar, but still distinguishable.
In Judaic and Christian texts, Seraph are celestial beings, or angels in some traditions. Their name means "burning ones".