If someone gets engaged and I have information that the other person has serious character and personality flaws am I obligated to tell that person

This is one of the most important and practically relevant questions in the laws of lashon hara (forbidden speech). Yes, you are not only permitted but obligated to share relevant information that could protect someone from a harmful marriage — provided you follow strict conditions. This falls under the category of lashon hara l'to'elet (purposeful speech for constructive benefit), and withholding such information may itself violate the Torah prohibition of "לֹא תַעֲמֹד עַל־דַּם רֵעֶךָ" — "Do not stand idly by the blood of your neighbor" [Leviticus 19:16].

Key Takeaways

You are obligated to warn someone about serious character flaws in their potential spouse — silence can constitute "standing idly by the blood of your neighbor."
The Chafetz Chaim codifies specific conditions that must be met before sharing such information, and violating those conditions can turn permitted speech into forbidden lashon hara.
The information must be firsthand, accurate, and serious — not rumor, exaggeration, or minor flaws.
Your intent must be purely protective, not motivated by personal dislike or desire to break up the match.
This is a nuanced and high-stakes area of halacha — consult a rabbi before acting, especially in complex cases.

Detailed Answer

The Core Tension: Two Competing Torah Obligations

There is a fundamental tension here between two Torah principles:

The prohibition of lashon hara — "לֹא תֵלֵךְ רָכִיל בְּעַמֶּךָ" — "You shall not go as a talebearer among your people" [Leviticus 19:16].
The obligation to protect others from harm — "לֹא תַעֲמֹד עַל־דַּם רֵעֶךָ" — "You shall not stand idly by the blood of your neighbor" [Leviticus 19:16].

The Torah places both commandments in the same verse, hinting at the constant need to balance them carefully.

The Chafetz Chaim's Framework: Lashon Hara L'To'elet

The Chafetz Chaim (Rabbi Yisrael Meir Kagan, author of Sefer Chafetz Chaim) is the primary halachic authority on these laws. He rules that speech which is normally forbidden as lashon hara becomes permitted — and even obligatory — when it serves a genuine protective purpose (to'elet).

He lays out seven conditions that must ALL be met:

You witnessed the matter firsthand — not hearsay or rumor
You assessed the situation carefully — and concluded it truly constitutes a serious flaw, not just a personal pet peeve
Your intent is purely to protect — not out of hatred or personal interest
You first attempt less harmful means — if the person could be warned without revealing details, do so
You do not exaggerate — report only what you know to be true
The potential harm from sharing must outweigh the harm to the subject's reputation — a genuine calculation is required
The information will actually be useful and acted upon — not shared in vain

[Chafetz Chaim, Be'er Mayim Chayyim, Laws of Lashon Hara, Principle 10]

What Counts as "Serious" Enough?

The Rambam (Maimonides) in Mishneh Torah emphasizes just how severe lashon hara is in general:

"שְׁלֹשָׁה לָשׁוֹן הָרַע הוֹרֶגֶת — הָאוֹמְרוֹ, וְהַמְקַבְּלוֹ, וְזֶה שֶׁאוֹמֵר עָלָיו" — "Lashon hara kills three people: the one who says it, the one who accepts it, and the one it is said about."

[Mishneh Torah, De'ot 7:3-5]

This underscores that even when speech is permitted or obligatory, both speaker and listener must handle the information with extreme care and not "accept" it as absolute truth — rather, they should investigate further before acting on it.

In the context of shidduchim (matchmaking), the Chafetz Chaim rules that the following qualify as serious enough to warrant disclosure:

Violent temper or abusive tendencies
Chronic dishonesty or deceptive behavior
Serious mental illness that would affect the marriage
Hidden addictions
Prior conduct that would be halachically or practically devastating to the other party

Minor quirks, differences in personality, or matters of personal taste do not meet the threshold.

The Obligation of the Listener

Importantly, the person receiving the information is not permitted to fully "believe" it as established fact. The Chafetz Chaim rules they should:

Take it seriously enough to investigate — not ignore it
Not act as if it is proven — do not break the engagement based on one person's word alone without further inquiry
Not spread the information further

This protects against the warned party becoming a vehicle for additional lashon hara.

A Broader Torah Theme: Speech as Life and Death

Proverbs 18:21 frames the stakes powerfully:

"מָוֶת וְחַיִּים בְּיַד־לָשׁוֹן" — "Death and life are in the hand of the tongue."

[Proverbs 18:21]

In the context of shidduchim, this is literally true — a marriage entered in ignorance of serious flaws can lead to a lifetime of suffering, while timely, accurate, carefully delivered information can save a person from tremendous pain.

Practical Bottom Line

You should speak up if the flaws are serious, firsthand, and your intent is protective.
You should not speak up if the information is rumor, minor, or if your motivation is anything other than the person's welfare.
How you say it matters — be factual, calm, and limit the information to what is truly necessary.
If you are unsure whether your information meets the threshold, or how to communicate it, consult a qualified rabbi or posek before acting.

For personal guidance, consult your local rabbi or posek, as this is a highly nuanced area of halacha where individual circumstances matter greatly.