I'm not new to FEH, but I've never been too interested in this till now. I'm gonna build up an Odin just cause I can, so if I gave Odin SPD/DEF Bond would the buff carry over during combat?
Do you mean if Spd/Def Bond will activate his bladetome?
Or if Spd/Def Bond will work during combat?
--
Spd/Def Bond 3 will give you +5 to Spd/Def if you're adjacent to an ally. During combat, you'll have +5 Spd/Def; however, Odin's bladetome won't pick up those +5 as extra damage. Only visible buffs (when your unit's stats turn blue) affect bladetomes.
Bladetomes (and blade-like tomes like Odin's) only take in consideration field buffs for their extra atk. That means those buffs you can see in the battlefield (the blue numbers), given to the unit as hone/fortify, link (like Odin's refinement) or tactic skills to name a few.
Non-field buffs are those not visible, those which say "in combat", like Spd/Def bond. So Odin will get those 5 points of Spd and Def in combat, but those points will not be added to the bladetome attack.
Field buffs don't stack and you carry only the highest of them, and combat buffs can be stacked over anything. For example, you gice Odin hone attack (+4atk) and you trigger his tome (+6atk/spd), you will have just +6atk/spd ignoring the lower +4atk. But you can put Spd/Def bond over that +6atk/spd Odin, resulting in a +6 Atk +11Spd +5Def during combat.
I hope to have explained myself well.
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes