I'd like to explore more what St Thomas means by character in reference to the sacraments. This is from part III of the Summa, q. 63, a. 1.
First he says that the sacraments have two purposes:
1. They are a remedy for sin
2. They "bring the soul to its fullness in things pertaining to the worship of God in terms of the Christian life as a ritual expression of this."
He uses a comparison, saying that soldiers are marked off by some physical sign when they are deputed for a certain function. Similarly, "Since by the sacraments men are deputed to a spiritual service pertaining to the worship of God, it follows that by their means the faithful receive a certain spiritual character."
"God imprints his own character on us" through the sacraments, through this spiritual sign.
In this first article Thomas is simply saying that some sacraments imprint a character. He cites St. Paul: "He anointed us, set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come" (2 Cor 1:22).
Here I would just like to note the interesting reference to the second purpose of the sacraments. It has to do with offering worship of God. It seems that Thomas is thinking primarily of worship in the sense of liturgy. It might be possible, though, to extend that meaning a little in terms of how the baptized live their lives in the world.
"You also, like living stones, are being built into a spiritual house to be a holy priesthood, offering spiritual sacrifices acceptable to God through Jesus Christ." (1 Pet 2:5). These spiritual sacrifices can be anything in our life, all that we do and suffer for God. The traditional prayer of the morning offering expresses that reality. Our whole lives, offered to God in all their details, form a sort of liturgy of life. In that sense, life becomes liturgy, and I think it can be said that the character of baptism is very much involved with this.