About
About This Site
Relationship Diagram Generator (soukanzu.jp) is a free web tool launched in 2025 with a simple mission: to make visualizing human relationships effortless. From mapping out TV drama characters and novel plots, to building TRPG character sheets, family trees, organizational charts, and historical figure relationships — it works for anything that involves people and how they connect.
Since launch, the tool has been used by thousands of creators, students, and professionals each month, with tens of thousands of diagrams created in total. No registration is required. All your data stays in your browser's local storage, so you can use it with peace of mind even for unpublished creative work or sensitive organizational information.
Operator Profile
| Operator | TOMI |
|---|---|
| Role | Indie developer / Web tool creator / Creative-support service builder |
| Web Dev Experience | 10+ years (frontend & backend) |
| Specialties | HTML5 Canvas, JavaScript, UI/UX design, SVG and graph rendering |
| Site Name | Relationship Diagram Generator |
| URL | https://soukanzu.jp/ |
| Launched | 2025 |
| Contact | Contact form / contact@soukanzu.jp |
| X (Twitter) | @TOMI_AI_ |
Why I Built This Tool
The seed for Relationship Diagram Generator came from a personal frustration: while reading novels, I would often lose track of who was related to whom, especially in stories with complex family trees or large casts. Official drama and manga sites sometimes publish character relationship charts, but there was almost no easy way for individuals to make their own — for the books they were reading, or the stories they were writing. Tools like Excel or generic graph editors technically work, but they're far too clunky for the "I want to sketch this out right now while the idea is fresh" moment.
The other major motivation was supporting creators. On platforms like X (Twitter) and pixiv, novelists and fan-fiction writers often hit a wall when their cast grows past ten characters: keeping all the relationships straight in their head becomes nearly impossible, and conveying that complexity to readers is even harder. TRPG players face the same issue with NPCs and faction politics across long campaigns. I wanted to build something that fits naturally into that creative workflow — a thinking aid for moments when notebooks and text editors stop being enough.
I also wanted to serve viewers who enjoy unraveling drama and film casts at their own pace, without spoilers. Official relationship charts often show every character from the start, which can spoil arcs for someone watching mid-season. Building your own diagram, character by character as you watch, is a different kind of experience — and it's one of the experiences this tool was designed to make easy.
The reason I keep the core service free is because creative ideas are fragile. The instant a paywall appears, the idea often vanishes with it. So the core features — adding people, drawing relationship lines, normal saving — stay completely free. Higher-cost features like AI image generation and watermark-free high-resolution exports are bundled into a single ¥500 monthly plan (no auto-renewal, by design). Revenue from those plans covers servers, AI API costs, and ongoing development, which lets the free foundation keep working long-term.
Looking ahead, the roadmap focuses on improving AI diagram accuracy, expanding the gallery for user-shared works, polishing the English UI for international users, and adding richer relationship expressions (multiple lines, grouping, timelines). User feedback drives every priority decision — if you have a "I wish it could do X" thought, please send it in via the contact form.
Tech Stack
This is an indie-built site, but it runs on the same kind of stack you'd see in commercial services:
- Frontend: HTML5 Canvas, Vanilla JavaScript (framework-free), CSS3
- Hosting: Vercel (CDN edge delivery)
- OG image generation: Cloudflare Workers + Satori (dynamic SNS previews)
- AI features: Cloudflare Workers calling AI APIs, with KV-based rate limiting
- Gallery: Cloudflare D1 (SQLite-compatible) + Workers
- Payments: Stripe Checkout (premium plan)
- Data storage: Browser LocalStorage (your personal diagrams never touch our servers)
Key Features
- Intuitive drag-and-drop interface for building relationship diagrams
- 10-color palette with image upload support
- Bidirectional relationship display with auto-arrange
- High-resolution (3×) image export
- Social sharing to X, LINE, Facebook, and more
- Dynamic OGP image generation for link previews
- Built-in templates (Drama, Family Tree, Team)
- PWA support (works offline)
- AI-assisted draft generation from a story summary
- Public gallery for browsing and inspiration
Contact
For questions, feature requests, bug reports, or media/partnership inquiries, please reach out. I aim to respond within 3 business days.