Extract from Summa Theologica, IIIa, q. 72, art 5: Whether the sacrament of Confirmation imprints a character?
As stated above (Question 63, Article 2), a character is a spiritual power ordained to certain sacred actions. Now it has been said above (1; 65, 1) that, just as Baptism is a spiritual regeneration unto Christian life, so also is Confirmation a certain spiritual growth bringing man to perfect spiritual age.
But it is evident, from a comparison with the life of the body, that the action which is proper to man immediately after birth, is different from the action which is proper to him when he has come to perfect age. And therefore by the sacrament of Confirmation man is given a spiritual power in respect of sacred actions other than those in respect of which he receives power in Baptism. For in Baptism he receives power to do those things which pertain to his own salvation, forasmuch as he lives to himself: whereas in Confirmation he receives power to do those things which pertain to the spiritual combat with the enemies of the Faith.
This is evident from the example of the apostles, who, before they received the fulness of the Holy Ghost, were in the "upper room ...persevering ...in prayer" (Acts 1:13-14); whereas afterwards they went out and feared not to confess their faith in public, even in the face of the enemies of the Christian Faith. And therefore it is evident that a character is imprinted in the sacrament of Confirmation.