The Rakan build for Support is Shurelya's Battlesong and Guardian. This LoL Rakan guide for Support at Platinum+ on 13.23 includes runes, items, skill order, and counters.
Rakan Counters
51.3%
(-0.7%)
42.6%
(+0.4%)
8.1%
(-1.7%)
3.4%
(-0.2%)
239
(-7)