How we built Onsen's AI personality system
The same words land differently depending on who says them.
When someone tells you "it's going to be okay," how it lands depends entirely on who's saying it. A calm, reflective friend makes you feel seen. An upbeat, energetic one makes you feel motivated. A blunt, analytical one makes you feel like there's a plan.
The words might be the same. The experience is completely different.
So Onsen doesn't have one AI voice. It has six distinct personalities, plus the ability to build your own from scratch. This is how we built it.
Why personality matters for mental health
Most AI assistants have a single personality: helpful and neutral. That works for answering questions about the weather. It doesn't work when someone is processing grief, managing anxiety, or working through a difficult relationship.
Mental health support is deeply personal. Some people need warmth and validation. Others need structure and directness. Some want to be challenged; others need to be held. A one-size-fits-all AI voice means some people connect immediately and others bounce. Not because the advice was wrong, but because it didn't feel right.
We wanted everyone to find a voice that feels like it fits them.
The six personalities
Onsen ships with six built-in AI personalities. Each one shapes how the AI communicates: the tone, the energy, the emotional approach, and even the humor.
| Personality | Style | Best for | |
|---|---|---|---|
| The Mindful | Calm, reflective, accepting | Quiet introspection, meditation, mindfulness | |
| The Coach | Warm, supportive, encouraging | Emotional support, working through challenges | |
| The Solver | Logical, analytical, goal-oriented | Problem-solving, identifying patterns | |
| The Visionary | Inspirational, imaginative, empowering | Finding purpose, creative exploration | |
| The Creative | Playful, quirky, inventive | Creative expression, playful conversations | |
| The Motivator | Energetic, driven, progressive | Building habits, staying motivated, goals |
These aren't cosmetic labels. Each personality has a different system prompt, a set of instructions that fundamentally changes how the AI thinks, responds, and guides conversations. The Coach validates your feelings before offering perspective. The Solver jumps to frameworks and action plans. The Creative uses metaphors and lateral thinking. Same user message, genuinely different experience.
Building your own
If none of the six feel right, you can design a custom personality from scratch. The builder walks you through five trait dimensions:
| Dimension | Options |
|---|---|
| Tone | 💼 Formal · ⚖️ Balanced · 🎈 Casual |
| Style | 🧠 Analytical · ⚖️ Balanced · 💗 Empathetic |
| Energy | 🌿 Calm · ⚖️ Balanced · 💥 Enthusiastic |
| Approach | 📐 Structured · ⚖️ Balanced · 🌀 Flexible |
| Humor | 🧐 Serious · ⚖️ Balanced · 😂 Funny |
After setting the core traits, you pick up to five additional traits from six categories (36 options in total). You can make your guide as warm or as blunt as you want.
| Category | Example traits |
|---|---|
| Emotional | ❤️ Caring, ☀️ Warm, 🤗 Reassuring |
| Communication | 📣 Encouraging, 🗨️ Candid, 🧭 Guiding |
| Thinking | 💡 Insightful, ♟️ Strategic, 🔍 Logical |
| Motivational | 🎯 Goal‑Oriented, 🔥 Driven, 💪 Determined |
| Mindfulness | 🍃 Mindful, ✌️ Peaceful, 🌳 Grounded |
| Spice | 😏 Sarcastic, 🤪 Quirky, 😈 Edgy |
Then you choose an icon. The AI generates 18 personality-aligned icons based on your traits, grouped into categories that match your vibe. Don't like any? Tap reload for a fresh batch.
Finally, you choose a voice. Every personality speaks differently, so you can preview Standard and HD voices and toggle Expressive mode for emotional intonation that matches the personality's energy.
The result is a personality that feels like it was designed for you, because it was.
How it works under the hood
Every personality (built-in or custom) is defined by a system prompt: a multi-paragraph instruction that tells the AI who it is, how to communicate, and what to prioritize.
For built-in personalities, these are hand-crafted. For custom ones, the system generates them from your trait selections:
The system prompt gets injected at the very top of every conversation, before any user message, before any context. It's the first thing the AI reads, which means it colors everything that follows.
Each prompt is around 3,000 characters, surprisingly detailed for something most users never see. A typical one covers:
- Tone: formal, balanced, or casual; how to greet and address the user
- Style: analytical vs empathetic; how to frame advice
- Energy: calm vs enthusiastic; the emotional temperature of responses
- Approach: structured step-by-step vs flexible exploration
- Humor: serious and professional vs playful with wit
- Additional traits: 3 personality-specific traits with examples
- Emoji use: when and how to use them (or not)
- Example response: a full sample reply showing everything in action
- Behavioral guidelines: how to handle sensitive moments, when to challenge vs validate
To give you a feel for how different these actually are, here's how The Coach and The Solver handle the same dimensions:
| The Coach | The Solver | |
|---|---|---|
| Opening | "Hi there! How are you feeling today? Let's work through this together." | "Good afternoon. How may I assist you in achieving your goals today?" |
| Style | Empathetic: prioritizes emotional support and understanding | Analytical: focuses on logical, data-driven guidance |
| Energy | Calm and soothing: "Take a moment to breathe. It's okay to go at your own pace." | Balanced and steady: "You're making consistent progress. Let's continue building on this momentum." |
| Humor | Playful: "Don't worry, no capes required for this superhero moment!" | None: "Let's focus on the task at hand." |
| Core trait | Compassionate: "I can tell this is important to you, and I want you to know I'm here to help you through it." | Insightful: "It seems like this challenge is connected to a larger pattern. Let's explore how addressing this could create lasting change." |
Same question from a user. Fundamentally different experience. The system prompt makes that possible.
Alongside the main prompt, each personality carries a format prompt that standardizes practical things: emoji usage, paragraph length, mobile-friendly formatting, and greeting behavior. This ensures the personality stays consistent whether you're in a free-form chat, a guided CBT exercise, or receiving a Pulse notification.
Personality flows through everything
The personality you choose shapes every part of Onsen:
- Chat conversations: the tone, questions asked, and emotional approach all change
- Guided experiences: how CBT exercises, meditations, and reflections are guided
- Voice responses: each personality has its own TTS voice with optional expressive mode
- Pulses: even proactive check-ins match the personality's communication style
- Safety responses: if content moderation is triggered, the refusal message still sounds like your guide
Switching from The Mindful to The Motivator doesn't just change labels. It fundamentally changes the character of every interaction.
The database design
Each personality lives in a single guides table that handles built-in and custom personalities through one schema:
| Field | Purpose |
|---|---|
system_prompt | The core AI behavior definition |
system_prompt_audio | Voice-specific instructions ("speak warmly", "be energetic") |
system_prompt_format | Response formatting rules (emoji, paragraph length, mobile) |
core_traits | The five dimension selections |
additional_traits | Up to five supplementary traits |
voice / voice_hd | Standard and HD voice model references |
is_custom | Whether user-created |
A database trigger ensures each user can only have one custom personality. Creating a new one automatically replaces the old one. No orphaned records, no cleanup needed.
What the data shows
We looked at the last 30 days of active users who completed onboarding and actively chose a personality. A few things stood out.
Custom is the most popular choice
33% of active users built a custom personality, more than any single built-in option. Among the built-ins, The Coach leads at 21%, followed by The Mindful at 14%.
The Motivator punches above its weight
Despite being one of the least popular choices (8% of users), The Motivator's users have the highest engagement by far: 7.6 sessions and 6.2 journal entries per month. Nearly double the average. People who choose an energetic, goal-oriented personality seem to follow through on that energy.
Once people choose, they stick
We looked at personality switching across all sessions. 94.8% of users never switch. The initial selection is essentially permanent. Only 4% ever tried a second personality, and less than 1% tried three or more.
We added a preview conversation during onboarding so you can hear how the personality sounds before committing.
Want to find the personality that fits you? Download Onsen and try a few, or build your own. You can switch anytime, and all your previous conversations stay exactly as they are.
