Across 1,504 ranked games on patch 16.21 in Master and above, Rell mains win 54.9% of their games as support.
Their most common build appears in 45.4% of those games and wins 53.3% of its 683 games.
It starts 
Celestial Opposition and 
two Health Potions, completes 
Zeke's Convergence, 
Locket of the Iron Solari and 
Knight's Vow in that order, and takes 
Ionian Boots of Lucidity.
Two other builds are played often enough as support to be measured on their own.
As support, Rell mains fare best against 
Lulu (won 67.6% of 68 games), 
Nautilus (won 59.6% of 52 games) and 
Seraphine (won 71.4% of 14 games).
Their hardest matchups, the champions that counter Rell, are 
Alistar (won 37.7% of 114 games), 
Leona (won 37.1% of 35 games) and 
Bard (won 38.2% of 34 games).