Compatibility is a phenomenon within the world of Aileron. In short, it encompasses an individual's potential to form powerful and lasting bonds with other individuals.
At its root, compatibility refers the to ability and capacity for one individual to form a relationship with another individual. This involves friendships, romances, rivalries and any type of relationship that can exist between two people.
The greater the compatibility between two individuals, the stronger potential and power the two individuals can reach while together. Conversely, the poorer compatibility, the less likely two individuals will be able to achieve such harmony and synchronization. This particular applies to Beast Fusion, which requires that the two participants be in full and perfect synchronization. Should two individuals with poor compatibility attempt a Beast Fusion, this would result in a loss of control and transform into Chaos Beast Fusion, in which the individual would lose control and go on a rampage. As a result, compatibility is a crucial aspect that any potential user of Beast Fusion must take into account. It is said that should two with poor compatibility attempt a Beast Fusion or lose compatibility whilst in Beast Fusion, the surge of power and struggle could literally rip one's own soul apart.
If two individuals have a good compatibility with each other, it is still possible to remain out of sync and thereby unable to execute a True Beast Fusion. These "breaks" can result from something as simple as disagreements or anything that can cause a rift between the two parties. Once those breaks are repaired however, compatibility can be restored.