The most important thing to learn when playing Relius is how to manage Ignis. When you summon Ignis, her meter will passively drain and will drain in a greater chunk every time you perform a move that uses her, meaning you have to be frugal with your use of her. If you see the gauge about to empty, turn her off. Either knock them down first or just upback j.5D. Generally Relius's 5D moves are safe enough that you can do this without taking a serious hit, and it's always better to take a little chip damage than to lose your most valuable asset.
Additionally, it's good to know all the different ways you can get Ignis out. There is, of course, the 5D moves, but Ignis will also appear onscreen any time you perform a move that uses her (for extra Ignis meter cost) while she isn't currently called out. What this means is that you can use the following situations to bring her in, for example:
- Combo into Lauger, followup of your choice.
- Any oki special after a 3C
- 214A then dash up
Relius's offensive potential lies primarily in his ability to create very scary blockstrings and put constant pressure on his opponent. You can start offensive pressure with 214A as it will snuff a surprising number of attacks and it will put blocking opponents in a defensive state as you dash up behind Ignis to continue your assault. With Ignis out, you can pressure your blocking opponent by getting in their face and pestering them with 5Bs, 2Bs and other normals combined with 6D and continuing to dash forward as Ignis makes your blockstring safe. Be wary of Counter Assaults though, as getting hit will cause Ignis to deactivate, effectively ruining your pressure.
In addition to his ground pressure and 214A, Relius has three very good air normals which he can use as a means of starting pressure. j.5A scales pretty badly, but is the safest and due to its multiple hits can be pretty reliable. You can think of it as a significantly inferior version of Kokonoe's j.5A.
Given that his offense is so scary, it's not surprising that Relius suffers defensively. He doesn't have very many reliable meterless reversals, so your best bet is usually to just learn how to block what you need to block and to punish accordingly. Due to its fairly fast startup and decent range, 5B can be a lifesaver when attempting to punish blocked attacks, just be careful not to get frame trapped. 236A can be a good defensive tool if you are certain that you can punish the whiffed attack. If your opponent is attempting to air dash in on you, wait for them to be within vertical range and give them a 2C for their troubles and then continue with a combo for damage or 214A to get Ignis out and regain your momentum. If you have meter, Counter Assault or Req Vinum (gear trap) are a solid reversals but if you time them improperly (especially gears) you are going to be in for a bad time. Finally, if you are certain they are doing something stupid and unsafe, you can pop an Astral and 9/10 it will beat out whatever they do.
5B/2B, Ignis, or Geara Lugia+dash up