As this is the training tower, and enemy Heroes can have a large assortment of abilities they do not normally have when the player owns them, this Niles had a level 3 Axebreaker ability.
Level 3 Axebreaker states:
If unit's HP ≥ 50% in combat against an axe user, unit makes a follow-up attack and the foe cannot.
Unit makes a guaranteed follow-up attack when attacked at HP ≥ 80%.
The situation was as follows:
Niles attacked me with his bow, thanks to Hector's Distant Counter I was able to counterattack, then Niles attacked again and that was the end of combat.
At first I was really confused as I know when I am attacked at full health Armads allows me to counterattack twice.
Then I noticed Niles had Axebreaker and thought it made sense that I did not get the second attack.
But then I saw that Armads has the word "guaranteed" in it's ability text, so I am now even more confused.
Is there a specific ability priority, so we can know when one ability will trump another in situations such as this?