Based on the description, it should add a flat bonus to attack. However, upon capturing a High Orc and hatching it, I noticed that the ATK Bonus wasn't flat, but rather a percentage.
Without High Orc:
With High Orc:
The ATK Bonus at this lntimacy Level should only be a flat +10. However, the ATK was increased by +15 instead. I then attempted to equip a weapon to prove if the ATK Bonus was indeed a percentage instead of a flat value.
With +10 Tempest  (no active High Orc pet):
With +10 Tempest  (with active High Orc pet):
With a weapon, instead of just adding +15 ATK (compared to being unarmed), it instead added +31 ATK. I have not gotten the High Orc above Shy yet so I'm not certain if the values will increase along with its intimacy level.