Larry is one of several random encounters throughout the game. He can be recruited as a weapons vendor and sent to an allied settlement. When asked about his story, he says "People want something, Smiling Larry finds it and brings it to them. The whole Commonwealth knows me."
Larry may not offer the dialogue to recruit him if there isn't a weapons shop in the settlement.
When assigned to a weapons shop, Larry sells a unique ripper, called Reckoning, that gives 15% resistance to damage when standing still.
Larry can be assigned to a weapons emporium in a settlement which will upgrade it to a Level 4 weapons emporium.
Smiling Larry does not have a "Trade" option so you cannot reacquire weapons he may take during settlement attacks. Keep this in mind when having him in your settlement. He will not take weapons if he don't have the ammo for it, so storing weapons is safe as long as you don't store ammo.
pc If Larry disappears after being sent to a settlement, he may end up in the "Quick test cell" limbo which is a very small test room, with other random NPC such as Scribe, Ness, Mr. Gutsy, etc. Ness will be hostile and may possibly kill any non-essential NPC. If he disappears, use player.moveto 2f2a7 to find him and other NPC.[verified]
pcxboxoneps4 Some Level 4 Merchants won't travel to or stay at your settlement after being assigned there. Solution: after the merchant has been assigned, disable them and any other travelling companions they spawned with, such as accompanying mercenaries ("prid [ref id]", "disable"); fast travel away from the area they spawned in; then walk, don't fast travel, back to that exact same area and a different random encounter should occur, such as wild mongrels or synths; if so, re-enable the merchant ("prid [ref id]", "enable"); fast travel to your settlement; and finally, move the merchant to you ("prid [ref id]", "moveto player"). Now the merchant should stay put.[verified]
As of patch 22.214.171.124 Smiling Larry now shows up at his assigned settlement and works properly.
pcps4 It is likely that Larry will never appear at a settlement once hired.[verified]
It is possible to assign him to tasks by using the console: target him with prid 0002f2a7, enter setpv bCommandable 1, setpv bAllowMove 1 and setpv bAllowCaravan 1, then fast travel to a remote location and return, after that you will be able to command him using the workshop menu.
Another solution is to head to the settlement where he is wanted, type prid 0002f2a7 in the console to select Larry, then disable, enable, and finally moveto player.
ps4 It is possible for Smiling Larry to spawn dead. [verified]
pcps4 Sometimes the option to recruit Smiling Larry to your settlement isn't there. Instead, it is replaced with the 'curious' option when speaking to him. The same problem could also apply to The Scribe. [verified]
On PC, this problem can be worked around by traveling to the settlement at which you want to recruit Smiling Larry; using prid 0002f2a7, moveto player, Addkeyword workshopallowcommand, Addkeyword workshopallowmove, setpv bcommandable 1, setpv ballowmove 1, setpv ballowcaravan 1; "moving" Smiling Larry to the settlement using the workshop menu; and when the option becomes available after a short while, assigning Smiling Larry to a task using the workshop menu.