I would describe the speaker's personality as playful and humorous. He is being facetious in his response to his own statement:
Something there is that doesn't love a wall,
That wants it down.' I could say 'Elves' to him,
The speaker has a good sense of humor. He is mischievous in his line about elves desiring to tear down the wall. The speaker humors the neighbor in helping him mend the wall. He realizes the neighbor is old fashioned in his idea of having a fence and can't be convinced that the fence is unnecessary. The speaker is charitable in his efforts to help his neighbor mend the wall. He has a good will and a friendly personality.
The speaker does not have to help his neighbor mend the wall, but out of acts of kindness, he humors his neighbor. Although the speaker does not necessarily believe the wall is necessary, he goes to great lengths to help his neighbor rebuild the wall. No doubt, the speaker playfully criticizes his neighbor in good jest. While he considers him old fashioned, he complies with his request to mend the wall, even though the hunters and animals will tear it down again:
I see him there
Bringing a stone grasped firmly by the top
In each hand, like an old-stone savage armed.
He moves in darkness as it seems to me~
Not of woods only and the shade of trees.
He will not go behind his father's saying,
And he likes having thought of it so well
He says again, "Good fences make good neighbors."
Truly, the speak has a good spirit. He is playful and a bit mischievous. He is totally tolerant of the neighbor and his idea that "good fences make good neighbors." The speaker is a warm, caring man who does the right thing by respecting his neighbor's wishes.
I believe the speakers personality is that of bitterness and frustration. Because he does not understand why he must comply when no one is giving him a legitamate answer--as to why he must continue to rebuild the wall. He finds it to be a nuisance and pointless--ultimately he's missing the point--therefore, his personality is seeded on ignorance.