The Seraphim

Seraphim quoque similiter multitudo est angelorum, qui ex Hebraeo in Latinum ardentes vel incendentes interpretantur. Qui idcirco ardentes vocantur, quia inter eos et Deum nulli angeli consistunt; et ideo quanto vicinius coram eo consistunt, tanto magis luminis claritate divini inflammantur. Vnde et ipsi velant faciem et pedes sedentis in throno Dei; et idcirco cetera angelorum turba videre Dei essentiam plene non valent, quoniam Cherubin eam tegit. Sanctus Isidorus Hispalensis, Etymologiae, Liber VII, Caput V Source: Migne PL 82.273d-274a

Likewise the Seraphim are a multitude of angels whose name translated from Hebrew is 'The Flaming Ones'. They are called 'The Flaming Ones' because there are no angels set between them and God, and therefore as they are much more closely stationed to His presence, so they are much more inflamed by the brightness of the Divine light. Whence they veil the feet of the one who sits on the throne of God, 1 and therefore the other groups of angels are not able to see fully the essence of God, because the Seraphim cover it. Saint Isidore of Seville, The Etymologies Book 7, Chap 5 1 Isaiah 6.2