New International Version
But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness,
G2590 karpos fruit καρπός
(a) fruit, generally vegetable, sometimes animal,
(b) metaphor: fruit, deed, action, result,
(c) profit, gain.
καρπός emphasizes on the end-product.
On the other hand, 1 Corinthians 16:
15a You know that the household of Stephanas were the first converts in Achaia, and they have devoted themselves to the service of the Lord’s people.
G536 aparché ἀπαρχή
the first-fruits, the earliest crop of the year
This word is made of two parts: apo and arché.
apo means from.
arché means beginning
ἀπαρχή emphasizes the beginning and not the end-product.
καρπός stands in contrast to ἀπαρχή.
23 And not only the creation, but we ourselves, who have the firstfruits of the Spirit,
i.e., the first crop of the Spirit
groan inwardly as we wait eagerly for adoption as sons, the redemption of our bodies.
How might these "firstfruits" compare with the "fruit of the Spirit"?
They are different concepts. The fruit/καρπός of the Spirit points the end-products (love, joy, peace, etc.) produced by an individual Christian. The firstfruits/ἀπαρχή of the Spirit refers to collectively the first crop or the first batch of converts as a set first produced by the Spirit.