Baptized and confirmed and received the Eucharist.
Living an upright life.
No penalties (e.g., left the Church, etc.). Therefore, a Catholic who has left the Catholic Church cannot be a sponsor, and cannot be a "Christian witness" if they join another Christian communion.
Cannot be either parent
Age—exception:
The pastor or the minister of baptism may admit a Catholic who is younger than sixteen.
In this case, the Catholic who is younger than 16 must meet requirements above (i.e., full initiation, etc.).
A"Christian Witness" is a Baptized non-Catholic with Catholic sponsor
The other sponsor may be a baptized Christian of a different faith. This person is a "Christian witness" (and not a "sponsor").
The other witness with a Catholic sponsor may never be non-baptized. For example, Jewish or Muslim godparents are not permitted by universal church law.
From the 1983 Code of Canon Law Canon 874, §1: To be admitted to the role of sponsors, a person must:
Be designated by the one to be baptized, by the parents or the one who takes their place or, in their absence, by the pastor or minister and is to have the qualifications and intention of performing this role.
Have completed their sixteenth year, unless a different age has been established by the diocesan bishop or it seems to the pastor or minister that an exception is to be made for a just cause.
Be a Catholic who has been confirmed and has already received the sacrament of the Most Holy Eucharist and leads a life in harmony with the faith and the role to be undertaken.
Not be bound by a canonical penalty legitimately imposed or declared.
Not be the father or the mother of the one to be baptized.