They have unique, powerful Skills and boasts the best stats when maxed out. Having an entire party composed of SSRs is your main goal though getting them can be tricky since they have the lowest appearance rate in Summoning. Thankfully, the game is generous enough to give away weekly SSR tickets by just playing the game daily. Crystals are also reasonably manageable to acquire so saving up for 200 crystals (and hoping for the best) for a batch summon can be done without having to spend anything at the game.
These are the middle ground characters who can hold off on their own or perform closely with SSR characters. High-leveled and properly-maintained SR characters can be very reliable, even capable of outperforming some SSR characters. Since they have a better appearance rate than SSRs, you can easily obtain multiple copies of them and use those copies to enhance the original’s Skills.
While R stands for “rare”, these normal heroes can be quite common. It is recommended to upgrade them to 6* then max their level (LV50) so you can get 10 Crystals for maxing them out for the first time. R heroes can be considered as expendable fodder especially after maxing them out save for certain healers and tanks.
So, with the limited hero slots, which heroes do you keep? The obvious answer will be SSR characters because of their sheer power advantage and skill set. However, that doesn’t mean that you can just ignore the SR characters or even certain R characters. Their advantage lies on their commonality; since you can easily get multiple copies of them, you can easily enhance their Skills a lot faster compared to SSRs.
While you’re building your main SSR party, it’s still a fact that you’ll need a large pool of high-leveled SRs and Rs as reliable placeholder units to help you clear dungeons and complete your tasks. Healers and tanks are hard to come by so keeping SR and R characters that can perform these roles early on will be advantageous to you in the long run. Also, having a good roster of strong SR and R heroes can help you take advantage of damage and buff bonuses on specific dungeons that require specific characters in your party.