Jesus is fairer, Jesus is purerFairer and purer are used here for emphasis. Similar words are used in poetry to give impact.
Merriam-Webster's dictionary solidified my understanding. It defines fair as, "pleasing to the eye or mind especially because of fresh, charming, or flawless quality."
Think of the "fairest of the fair". The beauty queen. The best of all of the beautiful women. Fresh, charming, flawless. The second stanza reads:
Jesus is fairer, Jesus is purer who makes the woeful heart to sing.That's what the hymn's author chose to call Jesus. He is flawless, our perfect Savior.
1 Peter 1:18-20 says:
18 For you know that it was not with perishable things such as silver or gold that you were redeemed from the empty way of life handed down to you from your ancestors, 19 but with the precious blood of Christ, a lamb without blemish or defect.
At Passover, the Jewish people were required to slaughter a perfect lamb, one "without defect" (Exodus 12:5), and sprinkle that blood on their doorposts. Then the Angel of Death wouldn't visit them. They would be saved from certain death.
That's what our Perfect Sacrifice did for us. Jesus, the fairest, offered Himself as our sacrifice and saved us from certain death. If He wasn't the fairest, the perfect one, His sacrifice wouldn't have been acceptable to God. The Passover in Exodus foreshadowed our Fairest Savior's death on the cross.
Verse four gives a wonderful response to this news:
Beautiful Savior! Lord of all the nations!Amen and amen.
Son of God and Son of Man!
Glory and honor, praise, adoration,
now and forevermore be thine.