|Matthew may not be a knight, but he can prove to be quite the tank if built for it. He has a deceptive amount of durability built into his stat spread between his moderately high HP and DEF, colorless typing which makes him only lose weapon triangle to raven tomes, and also his high SPD which prevents him from being doubled from anything that isn't a Brave Weapon. Rogue Dagger+ further synergizes with this idea, allowing him to get +5 DEF and +5 RES providing he had a combat before being hit in the round.
So with the basic concept out of the way, you've no doubt a few questions.
His base attack is 25 at 5*-- the writing should be on the wall here. If you want to have an offensive Dagger user, you kinda need all the attack you can get as daggers themselves aren't very pointy, and if you're using Matthew, why play to something that's clearly not one of his strengths? ORKOs just aren't what Matthew excels at-- you'd be better suited to try and build Kagero, or better, a Brave Bow archer for this. If you want a guy that can cut any opponent down to size so your friends can finish them off, that's what Matthew's here for, but to do this he needs to survive, which means you can't compromise any of his defensive stats, as he can't really do this if he's getting one shotted.
Okay, makes sense, but why +SPD?
Matthew's defensive stats are decent, but as he himself once said, he's no knight. If he's ever doubled, he's not having a good time. So ideally you want to minimize the number of opponents this occurs with. 34 may seem relatively safe, but with things like Darting Blow, Hone Speed, etc etc, he's just viable against a lot more opponents if his starting point is 37. Also, given his rather high defense, doubling more opponents will up his offensive output significantly as Ignis is really strong with that DEF.
Why Ignis? Why not Bonfire?
This build for Matthew has two "modes." The first round, where he needs every advantage he can get, and later rounds, where he's not fighting fair. Ignis doing a bit more front loaded damage with a sudden "HELLO THERE!" means you can occasionally ambush low def foes (mages) and waste them in one shot-- Bonfire just doesn't give enough damage to have a prayer of doing this even against low DEF opponents, as Matthew's base ATK is just too low.
Why would I use a Close Counter for Matthew?
For Matthew to apply his rogue dagger debuffs and buffs, he needs to hit back. Without Close Counter, any melee foe is getting a free lunch-- and his special is charging a lot slower against these types as well. It's really the only truly viable way to play Matthew without him being a very specialized counter unit. Whether that's worth burning a Close Counter on is your prerogative. Distant Defense 3 would be okay, though not necessarily cheaper, if you wanted to maximize performance as a counter vs ranged units.
Why not Poison Strike?
The idea behind Matthew's concept was a good one-- take some hits and whittle the enemy down, but lowering HP totals is the most impotent way to whittle a guy down in this game. Too many tools like Vantage, desperation, etc etc trigger on lowered hps. It's much much more effective to endure an attack, and then nerf the opponent to the level where your team is going to be massacring them with little effort. Or heck, Matthew himself can do it, as the cute part of all these stat drops are once his opponents have had all their stats lowered by 5-7, Matthew is suddenly competitive with them in a straight up fight. Provided he can survive the first attack without being crippled, he can now skirmish pretty competitively with most units, as they'll be perpetually debuffed.
Why Matthew at all?
You use Matthew if you want a tank that: 1. Can make any opponent who battles him manageable by dropping their stats. 2. Has no fears of the weapon triangle (save raven tomes). 3. Virtually guarantees death on the next round for anyone who initiates on him. 4. You really hate Brave Lyn. He's pretty good at stuffing her! (Not so good for Reinhardt unfortunately... can't win em all...). While in theory, any dagger user can do this, Matthew has the best combination of defensive stats (and Rogue Dagger+ out of the box is nice too) to make it viable. All others have speed or hp issues, allowing them to get one rounded by too many characters, or just don't have high enough bases on def or res that it makes any sense to even try.
You'll notice some waffling on the B/C skills. Those are best played by ear depending on what you need-- Seal Atk 3 + Atk Smoke 3 is generally best, but if you're near speed breakpoints (or don't wanna burn a Brave Lyn), then you may find success with some combination of Seal Spd, Threaten Speed, or even Threaten Attack. Whatever works for what types of teams you tend to run.
Matthew with this build is viable in all game modes. Infernal Maps, Tempest Trials, Arena you name it. As long as you aren't on a map with constant flooding of reinforcements that must be killed in a round, he should serve you well. Pairing with a healer (or with clever use of his Reciprocal Aid, which plays to his strengths better than the movement skills as he's ideally up front) and possibly Spur Def/Res, he can hold a frontline near indefinitely, allowing your other characters to pick the bones with ease.