what character is the best at soloing and pve?
It depends on what kind of staging your going to be doing : Raiding, Mything or general staging; but the answer is pretty much the same.
Ryan, Yuki, Arien: All of these characters require 89 G coins to buy, which works out to about 8 dollars~9 dollars American ;-;.
Basically Ryan; is a killing machine, he's extremely agile, and you'll be able to dodge with him easily. Ryan's combos are extremely easy to learn, and almost require no skill at all; just a little bit of timing. All of Ryan's attacks are very very powerful, and when rebirthed, become even stronger; Such examples are his Amplified rail gun, and true Amplified rail gun attacks, these both hit for about 200 damage each; with certain amounts of points invested; but both these attacks pierce through enemies and push them back; allowing up to a total of 15 hits alteast. Ryan is considered an 'easy' class by many, and is a good pick if you're not confident in soloing.
Yuki is the master of Ice attacks, every single one of her attacks his 'Cold' element, this is good and bad; for example in Bonus 3 and Raids such as Lower and Upper jilly(volcanic), her damage multiplies either by 1.5 or 2x extra damage, which is insane, and in bonus 1; orcs take double damage from her attacks too! Yuki to be honest is a power house, she combines the power of a Ryan or Arien, and the ability to recharge mp at a fast rate; to create an extremely powerful class. IMO she's the best bossing class because she has the ability to freeze, and use Ice Dragons Heart, and the Rebirthed Version; to maximize hits on a boss monster; hitting up to 30 times each; and which, in myth with 3k str; is about 1.5k~1.7k per hit(I've seen (N-IDH) hit up to 70 times on mobs in myth). Yuki has many easy doable combos, and the easieset infinite I've ever tried. Play Yuki if you want to be amazing at bosses, and soloing against mob monsters.
Arien is a very unique character; she can command both fire and lightning elements, which make her extremely strong. I have personal with Arien, and I can say that they're a class you should try out. Her combos are difficult to say the least, but doable with a bit of practice. Arien has numerous skills that pierce, and have a damn fast cast time; example are Violent Strong Blast, where you shoot out 9 arrows, in rows of 3; this doesn't pierce, but it's damage racks up easily. She has many mobbing moves, which have a 'cast and walk away' time effect, such as calling thunder(and the Rb version) and most of the Flame attacks. What Arien lacks in dodging, and the ability to regenerate mana points; she makes up for in Her Critical buff, enabling her to have(at max) critical for 45+ seconds; and it's a party buff too; so if you ever want to party stage; you'll be loved. Play Arien if you want to be able to solo, but still be given a challenge.
I don't have much information on any other solo-able characters unfortunately, but I hope I helped.