Certain Legendary Heroes have the Pair Up ability, and careful use of this feature will help turn the tide of battle in your favor. For example, you can pair attackers and defenders together, cover weapon-triangle weaknesses, or protect staff-wielding Heroes with low Def. Additionally, paired-up Heroes will gain an increase of up to +4 to their stats, based on the stats of the ally they formed a pair with.
Legendary Heroes can be deployed on any map, but be aware that you can only use Pair Up in Main Story and Paralogue maps, Røkkr Sieges, Training Tower, and Allegiance Battles. Additionally, Allegiance Battles and Mjölnir's Strike even allow Heroes who aren't Legendary Heroes to form pairs.
There are some other advantages to forming pairs. In Allegiance Battles, for example, you can get a +10 bonus to your score when you use certain weapon colors to defeat others, so Pair Up allows you to increase your score by choosing different colors on the fly. Additionally, defeating foes with paired-up Heroes will add a +5 bonus to your score.