It is deeply unlikely that Ibn Saba was a real person. There are strong arguments against his existence, and the reports on him are weak and via a questionable reporter. 

It seems that these reports may have spread, despite their weakness, because they present a nice and cozy picture of early Islam, in which everyone is getting along and there is a convenient explanation for disagreement in the community by attributing it to a plot. This is desirable to those who want to see all the sahabah as perfect. However, in reality, there was a lot of fighting and disagreement during the first three caliphates, and afterwards, and the real picture is not nearly as comfortable.

The Shi'ite Encyclopedia (which is available on al-islam.org) has a very comprehensive discussion of the sources of these reports and their reliability and other factors.

Among Shi'i sources, Rijal al-Kashshi lists him as a person but rejects him. Anyway, this seems to be responding to the narrative that was popularized. As for the single mention of his name in al-Khisal in a dialogue, there is insufficient context to say anything meaningful about it, except that, were he taken as a real narrator, he would have been rejected, but most Shi'i scholars hold that he didn't exist.