Rider HP: increase
Rider DEF: increase
Rider ATK: increase
Dom and Reyn find themselves in the awkward position of being two of the strongest riders that provide bonuses to riders. Unlike fighters for example, most five star Riders provide bonuses to troops rather than Riders specifically making them terrible candidates for a rider troop formation. This then leaves you in the position of using—along with three star, Doc Gray—inferior heroes to form your troop formation. Unfortunately, the developers have left us with really very few options.
This is likely an oversight that will be corrected with further heroes later, or by design to counteract the advantage Riders have in speed.
At +5 Reyn provides a +50% bonus to Rider HP, +50% bonus to Rider DEF and +50% bonus to Rider ATK. Reyn should be evolved to 210 and used in your Rider formation.