here are some less-popular characters i like to work with:
Yahr - okay, I just like trying to make him useful since he has the unlimited-use-when-berserk Angry Dragon Rune, but honestly he's pretty average or even below average overall and requires a lot of setup to get some serious damage going.
But if you want to keep reading... he has Angry Dragon Rune and 2 free rune slots, 2 free skill slots.
I like to set him up with Magic Sword just because it's such a novelty with few characters who can get above double damage using it. However, if you want to use Magic Sword, you'll have to use up a rune slot (elemental sword rune of choice), both skill slots (Magic Sword only C & Zen Sword A), and of course elemental boosting equipment to bring the element of the Magic Sword rune to S. The Magic Sword setup would let him deal 2.25x damage for the entire round without penalty (you know, accuracy issues or take extra damage, etc). Angry Dragon has unlimited use while he's berserk, but unfortunately Fury Runes don't grant permanent berserk in this game... so you can either start with Fury Rune and just keep using it until it's gone (usually only 2 or 4 rounds only) or maybe give him Violence Rune to let him have a really huge payoff when he goes Violent, or a Double-Edge (but watch out because he is too average to survive Sun Rune using Double-Edge without support).
Yahr's pretty average in all stats except for a high SPD stat so he is not going to stand out unless you get a good setup like combining Magic Sword with a berserk status from a rune or a Shield Rune user, (Double-Edge,) and Angry Dragon... with his SPD stat you can expect him to hit twice normally. Yahr also has low LUK and unimpressive SKL so don't bother trying to boost his critical hit rate.
If you just want instant heavy damage with Angry Dragon for a few turns, it's the obvious Fury + Boost combination, although the Boost status can sometimes die before the Fury Rune's berserk - I haven't figured out if Fury's berserk lasts a fixed amount of time per character or if it has a small random range of turns for the character, but I do think some characters remain berserk longer than others.
Yahr can also do some magic and also equip magic-boosting skills (Magic A and Divine Right A), but you may want to leave it to the pros. His elements are also all C so you need to do some work to boost his effectiveness with anything.
Goesch - I love the Mow Down skill, and you will too! Goesch gets 2 free rune slots and 1 free skill slot. You CAN give him a Magic Sword rune setup that gives him 2.12x dmg, but since he's all about just doing regular attacks, how about a simple Crazed Rune and Double-Edge/Boost. He has high HP, so Double-Edge is pretty safe. Crazed is an acceptable choice because it won't really matter where he attacks when Mow Down activates.
Mathius - with 3 rune slots everyone agrees he's one of the best characters, but he's no Miakis/Kyle/Norma. Mathius is similar to Goesch but with an extra rune slot, less HP, and a worse Mow Down skill. Mathius has a lot of options, since he has a good MAG stat, but in this game, physical attacks rule. You CAN dedicate a rune slot to a Cyclone Rune (I think he even comes with one, nice) to help heal the party in bad situations, but his avg SPD isn't the best choice for a healer. Mathius can also use Magic Sword as well as the typical Boost/Double-Edge/Fury runes. He is a decent choice for Strike-Back (Goesch actually is not the best for Strike-Back because he has a very good EVA stat) since Mathius has an average EVA stat, and it's definitely enjoyable seeing him Strike-Back and Mow Down multiple times in a single round. He's not complicated but it's hard to decide what 3 runes to go with.
Josephine - she is obviously great with magic, but there are 2 things to note: good Magic Sword compatibility (Magic Sword A and Sacred Oath A = 2.38x dmg) and low ATK. Low ATK means Josephine should use a Power Rune to optimize her attack, not a double-damage rune. If your ATK is 10 and enemy's DEF is 8, Power Rune raises your ATK and you get 15-8=7 dmg. Doubling damage gives you 2*(10-8)=4. You do have to watch out for her low HP and light/weak equipment so you might end up just making her a mage.
Gavaya - this is my recommended character for Strike-Back. Gavaya has monstrous ATK and HP, so he can take it and dish it out. he also has poor EVA which makes him less likely to dodge a direct short-range attack to optimize Strike-Back opportunities. He's also S range, so stick him in in the very front to get the most exposure. his Freeze skill isn't going to be very beneficial with Strike-Backs because enemies will recover the next round (if they didn't DIE!).
For other runes, Gavaya doesn't have the best accuracy so I don't recommend Fury or Crazed. A Double-Edge Rune or Power Rune (one, not both together!) is perfectly safe if he has Eyeball Rings to heal himself after attacks. Boost is always safe if you have a plan to (fully) heal all your Boost Rune users when their HP drops to 1. He can also go with Magic Sword (A) for 1.94x dmg.
For general parties, I do recommend a Shield and Cyclone Rune in the party, although at the end, the Dawn+Twilight combo can replace Cyclone. it helps if the caster is really fast (primarily SPD but also Incantation skills). Don't forget that Shield Runes also boost your team's magic damage, so a heavy magic team still benefits.