Who is supposed to say the chatzi kaddish after the torah reading? Is it the person who read the torah or an ovel who is saying kaddish?
The baal koreh or gabbai says it as it is to separate parts of the davening rather than for a yasom.
Chatzi Kaddish - the Half Kaddish or Reader's Kaddish
During the morning service (Shacharit) Chatzi Kaddish is recited by the prayer leader (usually the rabbi or cantor) after the P'Sukei D'Zimra section of the service, after the Amidah prayer, and after the Torah service as a means of demarcating different sections of the service. During the afternoon and evening services it is recited before the Amidah. All versions of the prayer include the Chatzi Kaddish.
The “Half Kaddish,”or chatzi kaddish (Heb. חצי קדיש).
This is composed of several lines beginning with, “May [G‑d’s] great name be exalted and holy…” The half kaddish is recited by the one leading the prayer, and after each stanza the congregation says, “Amen,” which means “correct,” or “it should be that way.” In the middle of the prayer, the entire congregation says in unison, “G‑d’s great name should be blessed forever and for all eternity.”
The half kaddish is said when the community completes certain sections of the prayer service, yet they have not completed the prayer service. For example, we say it between the “Verses of Praise” and the blessings before the Shema prayer, as well as after reading from the Torah Scroll, and so forth.
. See Rabbi Schneur Zalman of Liadi, Code of Jewish Law, Orach Chayim 55:1.
According to this article:
The Kaddish recited after Keriat Hatarah does not especially "belong" to the Ba'al Keriah. Anyone who is a hiyuv (one who is obligated to say Kaddish that day), may insist on reciting this Kaddish, as well.
In fact, in many shuls that I have attended, a chiyuv - someone who observes yahrtzeit or is in his 1st 11 months of mourning - is the one who says Kaddish after Torah reading and NOT the Torah reader.