Corwell is a free unit obtainable through the Silence of the Songbirds event as a reward. The event only gives about 250 shards, which is LB3. The rest of the shards will have to be obtained from summoning.
As a unit, he has pretty high AGI and he's able to get a lot of accuracy through his skills + bow of choice. Mid~High skill modifier damage and they come with good effects. The Gunner and Nightblade sub job lets him hit harder and further if you want to sacrifice some of his evasion rates. He can be a pretty good answer to evasive enemies in general, but some of his moves can be quite costly. He can definitely be built as your main damage dealer if you have Illusory Bell.
3 bow of choice depending on how you want to build him:
Hunting Bow (TMR) is high accuracy + chance to lower enemy's evasion from normal attacks.
Platinum Bow has defense penetration up to 20 at +5. Highest base attack by going assault, or mid base attack with 18 crit by going crit type.
Killer Bow for 15 missile attack at +5. Highest crit rate stat but lower ATK than platinum bow.
He can be worth summoning if you need a unit against evasive enemies, or when the current content/meta leans more towards evade. His EX upgrade skills offer some decent benefits, but they are fortunately not crucial.
Job lvl 19 upgrades Target into a range 1 area buff that also gives up to 30 accuracy. A good buff to use against evasive enemies, but Aimed Fire can be generally better. Job lvl 22 simply lowers AP cost of his Dispel Spread by 8. Job lvl 25 new skill is only a guaranteed hit attack that deals high damage for only 28 AP.