Tol Ren.
The main problem Tol Ren is his hp and mods. It can be described as a "glass sword". His main damage boost comes from his 2nd skill, which increases the damage by 10 for next attack. The point is that 144 hp is not enough to be in a battle and he is extremely dependent on his skills, the loss of each of which is very strongly reflected in your performance. He has 2 main builds of mods: for tanking and for max dmg. Worth to notice, that the 3rd and 4th skills without dmg buff mods don't have enough damage and +10 dmg from 2th skill only makes their damage equivalent to the attack of the rest FP. I have no complaints about his Ult, it is still possible to do 100 damage for one target under all buffs and might.(+dmg mod on ult).

Now i will tell about his mod builds for 2 gamestyles:

1) Max comfort and survival. With this strategy we are using haste/honed blade(making every slash for 30 dmg), +20 shields on 2th skill, -2 cd of 3th, 10 shields on 4th and any mod on ult. Catas; Second Wind, Fade, Unstoppable, WIth this build we can use our 2th skill+Second Wind and tank 1 shot from any FP without effect for us, get 30 heals and buff for 10 dmg. But this gamestyle style still depends on positioning and competent use of skills.

2) Max DMG. There we are using +3 dmg on 1st skill, +5 on 2th, nothing on 3th, +8 dmg/10 shields for 4th, +15% dmg or anything else for Ult. Catas: Might, Fade, Unstopp.
How you can see, now Tol Ren have more dmg than usually, but his hp doesn't allow to realize all this dmg.

To illustrate the problems of Tol Ren I will compare it with Lex.
1) Tol Ren makes 2 hits: horizontal slash for 28 dmg with range 1 square and vertical hit with range 3 squares and same dmg. If enemie gets bot hits=34 dmg. Usually we can't deal dmg with horizontal hit, so we are doing only 28 dmg.(30 dmg with mod).
Lex also has 2 flamethrowers with a range of 3 sqaures and good coverage, which allows you to control dashes and catas. Also, if both flamethrowers hit a target, then it takes +5 damage = 32 damage, which is almost the same as Tol Ren. Conclusion:Attack Lex is more convenient and effective in use.
2) Almost players on Lex are trying to maximize survivability with such mods. These mods: +10 shields for target of 2th skill, +8 shields for 3th skill per hit/+20 shields for 4th skill and 99% of player using +10 heal on ult. Catas: Second Wind, Fade, Unstopp. There is can be only healing cata, because she is don't have skill with big dmg and all her dmg is going from dmg stacks, control and being in fight. Conclusion: Lex survives and tanks better than Tol Ren.
3) Dash. The dash of Tol Ren is very situational, as it has a small scope for striking, but it allows to move after dash (4 turns cd). The dash of Lex does more damage, deals aoe dmg in circle and recovers only 3 turns (also do not forget about the 4th skill, which allows you to compensate for the impossibility of movement after dash, blocking and exchanging part of the damage)
4) Control. Ult of Lex also allows 2 turns to control the catalysts and dashes, causing them 5 damage per turn and show their positions. Also, her ult is requires only 50 energy, tha is why Lex turns into one of the best characters to control. Tol Ren doesn't possess such features. However, the ult of each character is special and there is nothing wrong with that. It's just another Lex superiority over Tol Ren.

Tol Ren is still playable and funny character, we can still making a good games on him and good stats, but he is more weaker than Lex and very situative character, which need some boost,