Remus might have let out a hint of a smile on his face (although still generally absent) and some breath when his eyes landed upon the clean stretch of skin this Neville had revealed to him. He stared at it hard, just in case a hue of black would show itself if he tried to blink but no, nothing came out, it was all flesh. "You know about Death Eaters," he said, a little bit relieved, as he looked up to him, and it was clear in his features that he thanked Neville for reassuring him.
Well, this Neville was a Gryffindor and him flashing his clean arm must mean that he knew about the Death Eaters and their threat. Having learned that, he really didn't find reason to keep his distance from the man, anymore. At least, if he sat closer to him and Neville lied, he would be within arm's reach of strangling. So Remus thanked him as he occupied the space beside him and listened to his story. Their friendship didn't go so smoothly from there, though.
No, Remus never expected it to. When Neville had mentioned St. Mungo's, he held his breath again and he listened intently -- yet disbelieving -- to its significance in Neville's life...Merlin's beard, it all matched! He was a Longbottom, and Frank and Alice Longbottom are currently in St. Mungo's...!
He finally got it: the way this Neville's stare and cheeks were shaped, it reminded him so much of Alice Longbottom!
"S, so...so Frank died an old man," Remus began softly, having finally decided to (after cursing himself mentally) take a leap of faith. Lily had done it, so why couldn't he? "And there is hope for Alice." He looked glad to know of the news.
Pinning his left thumb tightly with his right thumb and trigger finger -- he was mostly making sure he didn't betray any sense of insecurity -- he muttered after a while, "Frank and Alice are some of my good friends." He smiled weakly at Neville, uncertainly. "But you have probably never heard of me..."
He looked at his hands as he paused for a solid moment...until finally, decisively, he said as he faced Neville: "My name is Remus John Lupin. I fought beside your parents once."