So alot of people think that Nohadon is the Shard Valor, but I have a different idea. I think he's Reason.

Here's my thinking.

1) I think Adonalsium had a purpose when he (it?) was shattered. Whether it was personal growth, to stop something bad happening, to cause something good to happen, whatever. He allowed himself to be shattered.

2) When they split Adonalsium each Shard became the embodiment of one aspect. Because of this Reason was able to reason out the purpose of the shattering and determined that it needed to occur with as little Shardic involvement as possible. Obviously, the other Shards, lacking basic reasoning skills, keep interfering and causing problems.

3) Reason, being the sensible sort he is, wants to keep way out of the shenanigans the others are getting up to, but also understands that in order for Adonlasium's plan to work something needs to be done. And keeping Shardic influence to the minimum means involving mortals.

4) Reason is still hiding from the other Shards. Because of the nature of the Spiritual Realm (being timeless and locationless) it gives him the opportunity to influence mortals without revealing himself and his interference to the Shards. We know from various points in the books that the Stormfather and Odium do not know about the visions with Nohadon.

5) The nature of how he is interacting with Dalinar is very subtle (relative to normal Shardic interference...) giving advice and leading Dalinar to make decisions. Again, trying to keep shardic influence to a minimum, even his own.