Paul mentions a brother of the Lord: well, 'the brother of the Lord'.
Carrier & Ehrman are debating it now via their blogs
1. Carrier's Argument 14 here (re the Ehrman-Price debate) -
http://www.richardcarrier.info/archives/11435
Also see parts of the Argument 12, which Carrier refers to in (3)
2. Ehrman's blog-post here
https://ehrmanblog.org/carrier-and-james-the-brother-of-jesus/
3. Carrier's blog response here -
http://www.richardcarrier.info/archives/11516
The links given above by Mcreal are extremely informative, hugely detailed (with copious references), and they deal very directly with not only all the pro-HJ points being raised in this thread (they are all the same pro-HJ claims made in all HJ threads), including what was said in Paul's letters as any reference to a real Jesus, and very specifically on the famous line where it says "other apostles saw I none, save James the Lords brother" ...
.... for which details and argument from Carrier - see the first above link-1, and scroll down to "Argument 12", where Carrier says that in the debate Ehrman repeated all sorts of completely false claims to say that Paul's letters made numerous references to Jesus as a real person (Carrier actually says that Ehrman was openly and repeatedly lying in his numerous claims about that), and in the 7th bullet point he (Carrier) explains what I referred to in a previous post a page or two back, where Carrier says that when Paul used terms like "brother ... brothers ... brethren ... sisters" and even "brother of the lord", in Paul's terminology that merely meant those who were correctly baptised into their faith.
And upon which point, see also the third of Mcreal's links (link-3 above), where Carrier replies to a response from Ehrman, and where he (Carrier) expands further on that same issue of James as "the lords brother".
As I said before - I do not know if Carrier is right to conclude that "brother of the Lord" simply meant an appropriately baptised member of the faith. But in the above described link and commentary from Carrier, he gives the reference for that discussion in his own book OHJ p582-592, so anyone here can read for themselves what Carriers reasoning is.
Though whether Carrier is right or wrong about the origin and meaning of "brothers of the Lord" (see also 1-Corinthians 9.5, which is the only other place where Paul mentions "brothers of the lord", i.e. plural in that particular letter), what is known apparently as "fact", is that -
1. Nowhere else does Paul ever again (in any of the letters), say that James (or anyone) was an actual family brother of "the lord". It is only that one single mention in Gal.1 19. It was never repeated again anywhere.
2. The structure of that sentence in Gal.1 19 has the form of a series of after-thoughts, as if the writer had first just written "other apostles saw I none" ... but then he (or someone else!?) remembered that he had seen someone called "James", and so he adds "save James" ... and then as a further after-thought in case his readers did not know who this particular "James" was, he then produces the explanatory addition of writing "...the lords brother" ...
... it reads as if the writer at first just meant to say "other apostles saw I none" ... but then he thinks, "oh I should add that I did see James" ... and then finally he thinks "oh, and I should explain that this James is a brother to the lord". It may be a coincidence that the sentence is constructed that way, but that sort of construction as if in a serious of "after-thoughts", might also be the result of latter additions by Christian copyists. That is - the sentence would actually have been complete if Paul had just written "other apostles saw I none". And it might be the case the the explanations of saying "save James", and then adding "the lords brother", were actually just later interpolations with copyist scribes, adding for clarity, what they believed happened (or what was meant) and who they believed "James" was.
Why might we think it's a copyist interpolation of that sort? Well firstly because of that after-thought style of the sentence (as just explained), secondly because we know that Christian copyists were in the habit of making such changes where they thought it better explained their later and changing beliefs, thirdly because half of all Paul's letters are now agreed to be complete forgeries anyway (so those were all 100% "interpolation"), and fourthly as I noted earlier (from Carrier OHJ, with references), several of Paul's letters apparently show quite clear evidence of being a composite pieced together from parts of quite different letters.
3. As Alvar Ellegard noted in his book (Jesus One Hundred Years Before Christ) - when Paul went to visit Cephas as described in that passage of Galatians-1 15-22, it was said to be 3 years after Paul's vision of Jesus; an event which completely changed Paul's life so that he dropped all of his earlier Jewish preaching and instantly began preaching Jesus as the messiah (that became Paul's entire life). And yet when he meets the actual blood brother of Jesus (i.e. this man "James"), Paul asks not one single thing about Jesus, and James apparently tells him not one single word about Jesus. That seems extremely strange, not to say highly unlikely, if Paul is finally meeting the actual brother of the messiah of God who has become the whole motivation for everything in Paul's entire life.
4. That same "James" apparently wrote his own gospel. And yet in that gospel, James makes no mention of being a brother of Jesus. In fact there is no claim that he even ever met any such person as Jesus!
So, by all the above am I claiming that Paul definitely could not have meant that this person James was actually a blood brother to Jesus? No. I am not claiming that. But what I am saying is that for all the reasons explained above, it is very far from clear whether that one line (never repeated again anywhere, and never confirmed by James himself), ever meant anything other than a brother in the faith, i.e. in just the spiritual sense.
On it's own that is, to put it mildly, extremely dubious evidence of Jesus as a real person. And yet, as I say (and as Carrier says in those above links ... and as Ehrman himself seems to accept), that one part of one line, is really the only thing that could be considered even remotely credible as actual evidence for anyone knowing Jesus as a real person anywhere in the entire NT bible.