So what happens when a 25mm base characters joining a 20mm base unit? Can they do it? Do they sit in the center and displace the guy behind them backwards if the unit has more than 1 rank? BAsically, can a sebeki character join an asar unit?

We just put them in the front corner.

It's like Chaos Warrior to Marauder to WFB5... It's not easy but you can do it. It the reason a base Harbinger on the same base than warriors.

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Dreadaxe
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This doesn't seemright since the rules specifically say characters have to go in the center of the unit. Also, I thought all harbingers were on 25mm bases...

The character would still class as being in the center of the unit, for game play purposes, but for the sake of your sanity and uniformity of your unit you can put him at the edge and tell your opponent he is attached to the unit.

Harbingers can be based on a "20mm or 25mm square base, according to the player's preference, Sebeki Harbingers are always on a 25mm square base" (pg 123, below profiles)

I hope that helps

To confirm what Hecate has said I have seen a number of players who have Harbingers on Both 25mm and 20mm bases the latter are particularily useful if you use chariots when a 25mm base may not fit in. The smaller bases can then be used when the Harbinger has joined a unit.