Well, the datamine is up:
-Unsurprisingly, Laegjern is a glass cannon. So is Gunnthra.
-Helbendi is shockingly tanky for a Red Sword Infantry unit. Also, his Red D Infantry gives him a whole 2 BST when he's not merged...
-Laevateinn is a slow Flying Green Mage. Even slower than Spring Camilla by a frickin 9-stat difference. However, she's has the highest ATK and DEF out of all the Flying Green Mages, with one more DEF than Spring Camilla, two more ATK than Bridal Sanaki and the 3rd highest RES that isn't Sanaki or Yune, but only 1 RES higher than Nino and Veronica. Laevateinn was made for a Gronraven/Gronserpent Build.
-Ylgr has high ATK and SPD, good DEF and decent RES. She is actually very comparable to Libra, as she has 2 more HP, the same ATK, +4 SPD and +2 more DEF than Libra, but -4 RES compared to Libra. Stat-wise, she's pretty good.
Also here's Ylgr's artwork.
I do like her shovel weapon, especially as it can double-in as a reference to snow shovels. Also, Ylgr's skills are Luna, Flashing Blade and Even DEF Wave 3.
As for the TT Seals, ATK/RES Bond is the best one, with Spur ATK/SPD as second best. Armoured Blow is just there.