Gwen

Gwen is the main character and you are forced to deploy her so obviously her availability is great. Her being in A tier might be a shock, but when you see the other people in this tier with her, I think it starts to make sense. Her decent mixed base and growth stats with her high Mastery make her really good in her class tree as the Gallant and Nightblade classes have some really decent scaling buffs that can be really helpful all throughout the game, but really shine mid to late game.

This is where a lot of people struggle with Gwen. 

Reaper - Reapers tend to tote a pretty high Mastery so using either the Lava or Mystic Ring on them should be a no brainer - especially when you see that Call to Arms power buff. The Greataxe is a really high power axe that has fantastic runes boosting everything a Reaper wants to do - hit hard with a crit. While 60 base accuracy IS  a problem there are multiple ways to fix it (Prophet Ring, Poison, Accuracy Buffs, etc). The playstyle for this Reaper makes it a very versatile class as you can buff units, wait, get your +40 crit that stacks as long as you don't see combat, go in with Blink and kill a high priority target or just Blink to finish objectives. Finally, Gwen's Brutal Strike makes her probably the best Reaper, complete with a Vampiric glyph. 

Ellisant - With the Lava (or Mystic) Ring, Gwen's high mastery stat in Ellisant allows her to buff herself or allies considerably. While the dip in Gallant vs Nightblade is just the question of do you want more power or more speed, Shadowstep for Gwen gives her some respectable dodge when paired with Blinding Light.
Additionally, the "Light" rune on the Conquest Bow and using either Brutal Strike or Full Draw skills with the Radiance Ring ensures nothing hits you as long as they don't have the "guaranteed to hit" empowerment - though it is very much overkill even for Demonic. 

Champion - Leveling her as a Nightblade gives her decent speed and the ability to double and dodge in Champion. Self casting Fleet Feet is extremely important for this build as it not only gives her speed to get in doubling/not getting doubled range, but also gives her 20 dodge (stacking) for 2 turns. Additionally, the Solemn Ring is a decent alternative ring with Gwen's defensive bulk where she can sustain with the Vampiric glyph on her personal skill.

Ancarant - Gwen really isn't that great of an Ancarant. She has the speed to not get doubled, has decent dodge, and can play a decent support role; however, she doesn't really have the power to kill anything. This can be mitigated a little by purchasing the Shiv from the shop once you have access to the "Great" weapons and putting the Crusader Rune on it, but still isn't a great option either. Another option she can try is using the Kris and going for the Piercing Rune which halves her already low power, but lets her ignore defense - so she will at least do some damage. Yet another option is to use the Advantage Ring to ignore 28% of defense and Antimage Ring to add a flat 3 to magic enemies. Finally, you can always use the Seeker's Enchant Weapon skill to add some damage to the class - it isn't terrible, just not great.