Affection is an attribute in Senran Kagura 2: Deep Crimson. It replaces the Flower of Balance system where you had to raise your Yin and Yang Masteries. Instead now you raise the Affection between two characters.
When selecting two characters, their current Affection level is displayed with a heart icon. You can also view the Affection levels with any character by pressing X on the Character Select screen and choosing the Affection Rate.
Each character has an Affection level with every other playable character (including themselves). Ultimate Asuka is considered to be the same character as Asuka, and Crimson Homura is considered to be the same character as Homura. All "Chibi" forms from the Waist-Deep DLC are also considered to be the same as their base character.
Affection starts at zero and has a maximum level of 50.
Affection is raised by performing Pair Battle actions with the characters.
These include the Tag Homing Dash, Tag Counter, and Combo Call, but the most effective way to raise it is to perform a Stacked Secret Ninja Art or Secret Fusion Ninja Art.
Using these actions against stronger enemies also seems to increase Affection more than against weaker enemies.
There is also a Shinobi Stone that increases the rate at which characters gain Affection.
Effects of Affection
The primary effect of Affection is that it raises the attack power of your Stacked Secret Ninja Arts or Secret Fusion Ninja Arts. These are already devastating attacks that can become overwhelmingly powerful with maxed affection.
As character affection increases for one another, what they say during the attack can also change. They may also change the name of the attack (ie. Kunai Rain being called Eight-Way Shuriken instead). Additionally, any characters that have a special intro or outro together will get more positive dialogue as their Affection increases.