Before us is Mishnah 13 in chapter 4 (in some editions it appears as Mishnah 11), and in it we meet Rabbi Eliezer ben Yaakov. It seems there were actually two separate sages by this name: one lived at the end of the Second Temple period, and he is the source of the mishnayos in Tractate Middos; the second was a student of Rabbi Akiva, and of him it was said, "the teaching of Rabbi Eliezer ben Yaakov is a kav and clean" - his Torah is measured and precise ("a kav"), and free of superfluous words ("clean"). He is the author of our mishnah.
"Ha'oseh mitzvah achas koneh lo praklit echad" - one who performs a single mitzvah acquires for himself one advocate:
Whoever performs a mitzvah - any mitzvah at all - acquires an advocate for himself. The word "praklit" is borrowed from the Greek (parakletos), and it means a defense attorney in court. The idea is that every act of a mitzvah creates an angel that accompanies a person, whose role is to serve as his advocate. The source for this is a verse in Iyov: "Im yesh alav malach meilitz echad mini alef lehagid l'adam yoshro" - it is enough for a person to have even one angel who speaks in his defense. And so it is with every good thing a person does.
"V'ha'over aveirah achas koneh lo kateigor echad" - and one who commits a single transgression acquires for himself one prosecutor:
Every single transgression a person commits creates a new prosecutor. The word "kateigor" also comes from the Greek (kategoros), and it means an accuser or prosecutor. It seems that Chazal borrowed these terms from Greek because the Jewish legal system, the beis din, is not an adversarial system: there is no prosecution and defense, but rather a panel of judges who investigate and ask the questions. For that reason Greek terms were needed in order to describe their court procedures.
The Zohar discusses at length how every evil deed a person does creates a "mashchis," a destructive force - and that is the prosecutor spoken of in our mishnah - which a person must then contend with. The reality of sin is an actual reality, and there is no escaping having to deal with it: if not in this world through teshuvah, then in the World to Come or in the intermediate worlds after his passing, where a person confronts his evil deeds, may Hashem spare us.
Beyond the personification of good and bad deeds, as though they literally create an angel and a spiritual force, it is worth noticing that the mishnah uses the word "koneh" - acquires - the person is its owner. This is not merely a record of good and bad deeds entered on some ledger, but the creation of something real - a force, an angel, whatever it may be - his defender or his prosecutor, and it belongs to him. In other words, it is his responsibility, for better or worse, throughout his entire life.
"U'teshuvah u'ma'asim tovim k'siris bifnei ha'puraniyus" - and teshuvah and good deeds are like a shield against calamity:
Teshuvah and good deeds serve as a "teris." The Gemara knows of terisim as a sort of window shutter, but the word "teris" is likewise borrowed from the Greek (thureos), and it means a shield - that round shield a soldier holds. Teshuvah and good deeds therefore protect a person from calamity. "Puraniyus" means bad things that come upon him, and not necessarily in the World to Come, but even in this world.
And we must be clear on this point, which we have already addressed and will return to again: basic Divine justice requires that every good deed be rewarded and every bad deed be punished. The way a person can protect himself from the consequences of sin is teshuvah, and that is what serves as his shield.
Some raise a difficulty: regarding the severe transgressions listed among the four categories of atonement - transgressions that carry kareis or the death penalty and the like - it is stated that teshuvah alone does not suffice, but rather teshuvah and Yom Kippur suspend the punishment until suffering comes, and in the end the calamity does come upon him. If so, what kind of shield is this if it only suspends the matter?
Some understand that this itself is the answer: teshuvah at least prevents the bad things from happening and reduces them, whatever the story may be.
And some understand that this has no bearing on our subject at all: teshuvah uproots the problem entirely, and prevents a person from having to face the consequences later on.
And what about good deeds? We cannot explain that a person's good deeds shield him from the consequences of his bad deeds - that is not the principle. The Rambam explains: suppose, hypothetically, that a person has no sins at all; through his good deeds he acquires merit, and as we find in various statements of Chazal, a person's merit protects him. One who has merits that protect him - when the rock falls, it does not strike his head but misses him, a kind of miracle that comes about through his good deeds. This is also the basis for the discussions of how bad things do not really happen to good people, because they are protected. This protection comes as a shield, as a guard that defends a person by force of his good deeds.
In summary: In this mishnah Rabbi Eliezer ben Yaakov teaches us that every mitzvah creates a defense attorney - an advocate before the Heavenly court - and every transgression creates a prosecutor, and a person "acquires" them and bears responsibility for them. We noted the Greek origin of the words peraklit, kategor and teris, and the words of the Zohar that every evil deed creates an actual destructive force. We further learned that teshuvah is the shield against the consequences of sin, while good deeds - as the Rambam explains - acquire for a person merit that protects him from calamity.