Center of the Battleline
From Archon Arcana - The KeyForge Wiki
A creature is in the center of the battleline when there are an equal number of creatures to both that creature’s left and right side.
There is only a center of a battleline if there is an odd number of creatures in that battleline. When there is an even number of creatures in a battleline, there is no center. If there is only one creature in the battleline that creature is in the center.
Related Topic: Battleline
—KeyForge Master Rulebook v17.4 September 2024
Commentary
If there is a single card in a battleline, it is both in the Center of the Battleline and on both flanks.
Source: Flank•Center of the Battleline