01— Good Welchi, inc. — Corporate
Building software people actually use.
A one-person studio led by a Flutter/Dart engineer with 6+ years of experience shipping mobile apps — from 0→1 prototypes to 10M+ MAU products. Incorporated as Good Welchi, inc. since 2020.
6+ yrs
Flutter / Dart in production
Full-time on Flutter since the 1.0 era (2019).
10M+
peak MAU on shipped apps
Peak MAU on a national fitness chain member app (2022–2023).
95/100
mobile security audit score
Independent 3rd-party mobile security audit (gold-tier scope, measured on a 2025 engagement). Covers data protection, transport security, obfuscation, and CI/CD hardening.
Elite
DORA Four Keys rating
Measured by client (GNUS, Inc.) across 5 repos (Jan 15 – Apr 15 2026, 90 days). The app repo I lead alone scored Elite — 1.94 deploys/day, 0.6% change-failure rate. Program total: High (3.0/day, 1.5%).
02About
Yuta Kawai / Founder
Good Welchi, inc.
Build small, grow long. Deliver it in a way that earns trust.
Good Welchi, inc. is a one-person incorporated studio founded by Yuta Kawai in 2020, specialising in mobile app development.
6 years of Flutter/Dart, paired with Firebase and AWS Cognito for auth, GitHub Actions and Bitrise for CI/CD, Unit/Integration/E2E test automation, and Sentry for monitoring — we design end-to-end, from development through operations.
From 0→1 prototypes to projects with 50-person teams and 200-person orgs — we have contributed as member, lead engineer, and technical advisor.
01
Maintainability ≠ compromise on UX
OOUI + Material Design + Clean Architecture — product that stays coherent as it grows.
02
Trust starts with fast replies
Frequent check-ins, psychologically safe reviews — trust is the first deliverable.
03
AI agents, used hard
Claude Code / Codex / Figma MCP daily. CLAUDE.md + git worktree cut effort 30%+.
03Services
Mobile apps — from design → implementation → operations.
01
Mobile App Development (Flutter)
iOS/Android apps built on Clean Architecture + MVVM. Designed for long-term maintainability.
02
CI/CD, Test Automation, Observability
GitHub Actions / Bitrise / Fastlane. Automated tests per PR, screenshot-based E2E evidence, Firebase App Distribution, Sentry monitoring.
03
Mobile Security
Transport security, data protection, obfuscation, CI/CD hardening — informed by a gold-tier independent 3rd-party mobile audit (95/100).
04
AI-Native Development
Claude Code, OpenAI Codex, Figma MCP in production. Guideline-driven AI workflows that cut effort by 30%+ without dropping quality.
05
Advisory / Review & Architecture
Flutter-specific judgement calls, architecture, review process — from spot advisory to ongoing retainer.
03.1By Role
Best slots: Lead and Advisor
— owning the app side of the stack.
lead
Lead Engineer
End-to-end ownership. Architecture, review culture, CI/CD — so the team can move fast without breaking things.
advisor
Technical Advisor
Flutter-specific deep dives, tech selection, architectural decisions — brought in where it counts.
builder
0→1 Lead
Small projects taken from idea to live product single-handedly — prototype through sustainable ops.
quality
Quality & Ops Foundations
CI/CD, automated testing, observability, security — raising the bar on legacy codebases.
04Works
Industry names only, for confidentiality.
Tap a row to reveal anonymised takeaways from that engagement.
2025–
Major religious org / learning app
Lead App Engineer · 10 / team of 30
Flutter・Dart・Cognito・Firebase・GitHub Actions
Security 95pt−
Takeaways
- —Led architecture foundation (MVVM × Clean Architecture) and turned it into the team-wide implementation baseline.
- —Built a new GitHub Actions quality gate — format, dart analyze, and Unit Test run on every PR.
- —Split Firebase App Distribution into dev/stg/prd with weekly auto-deploy, cutting device-verification lead time.
- —Designed the Sentry monitoring stack (ParallelWaitError unwrap, HTTP context, production noise filters).
- —Proposed removing sensitive payload fields from push notifications to cut unintended-outing risk on lock screens.
- —3rd-party security audit: 95/100. The app repo I own scored Elite on DORA Four Keys (1.94 deploys/day, 0.6% change-failure rate).
2025
Healthcare / wellness app
Member · 30 / team of 100
Flutter・Dart・Bitrise
+
Takeaways
- —On an Rx-based Clean Architecture codebase, added Unit Tests to core business logic before refactoring — improved maintainability with zero regressions.
2024–2025
Used-car retail / super-app (w/ BFF)
Full-stack member · 50 / team of 200
Flutter・Spring Boot・Cognito・New Relic
+
Takeaways
- —Owned the 'my car' registration API on the BFF (Spring Boot) side.
- —Took over the Flutter side mid-project when quality issues arose — led stabilisation and bug convergence.
- —Subsequently led the account-registration flow on Flutter — full-stack ownership across BFF and app.
2024
Municipal portal / resident app
Tech Advisor · 5 members
Flutter・AWS Amplify
+
Takeaways
- —Retained as tech advisor — front-end architecture and code review for the team.
2024
University research / agri scheduling app
Project Lead · 1 member
Flutter・Firebase
+
Takeaways
- —Sole owner — requirements → UI design → implementation → operations.
- —Added automated tests for every Cloud Firestore security rule, securing a research-originated product.
- —Chose Clean Architecture to absorb the frequent feature churn typical of research-to-product pipelines.
2023–2024
Major real-estate / industry SNS (mobile + web)
Lead Engineer · 5 / team of 20
Flutter・Firebase・PHP
+
Takeaways
- —Designed the architecture foundation including Flutter Web + mobile routing, establishing the team-wide implementation baseline.
- —Led 2 app engineers on an MVVM + Clean Architecture-based codebase.
- —Built push notification delivery on the backend in PHP.
2023–2024
Major energy / gas-station app
Member · 30 / team of 100
Flutter・Firebase・gRPC
+
Takeaways
- —Integrated a native iOS/Android marketing SDK into Flutter — no Flutter SDK was provided, so bridged each platform SDK manually.
- —Designed the complex permission-request flow involving push notifications and location services. Built branching logic based on existing permission states to ensure a bug-free consent dialog flow.
- —Planned feasibility studies, delegated tasks to team members, and managed overall progress while leading system-level design.
2022–2023
National fitness chain / member app
Member · 30 / team of 100
Flutter・Firebase・Bluetooth
MAU 10M++
Takeaways
- —Implemented Bluetooth integration with health watches and smart body-composition scales.
- —Took design ownership from the designer on features with OS-specific quirks (e.g. step-count sync).
- —Operated and evolved the GitHub Actions CI/CD environment over the engagement.
- —Ran code review on four axes — requirements, style, correctness, maintainability — with care for psychological safety.
2020–
Real-estate / condo management app
Lead Engineer · 3 / team of 15
Flutter・Firebase
+
Takeaways
- —Sole owner of the app side — design, implementation, and release (notices, facility booking, document requests, resident chat, disaster-prep).
- —Obfuscated code + encrypted secret storage (Keychain on iOS, Keystore on Android) + no debug logs on release builds.
- —Shared working TestFlight / Firebase App Distribution builds frequently to catch requirement drift early.
2019–2020
Entertainment / virtual-avatar app
Lead Engineer · 2 members
Unity・ARKit・OpenCV
+
Takeaways
- —Led avatar behaviour in Unity/C#, face tracking via ARKit/Dlib/OpenCV, and In-App Purchase billing end-to-end.
Interested? Let's talk.
Available from 1 day/week for spot engagements.
05Stack
Flutter at the core, extended across mobile, backend and ops.
Languages & Frameworks
Flutter6 yrs
Dart6 yrs
Firebase6 yrs
TypeScript4 yrs
Swift3 yrs
Kotlin3 yrs
Java2 yrs
Python1.5 yrs
C#1 yrs
Unity1 yrs
Spring Boot0.5 yrs
Infrastructure
GitHub ActionsBitriseFastlaneAWS CognitoAWS AmplifyGCP / BigQuerySentryNew RelicFirebase App Distribution
AI / Tools
Claude CodeOpenAI CodexFigma MCPNotionJiraSlackFigma
Certification
GNUS Network Platinum Engineer
GNUS, Inc. (awarded Jan 2024)
Top tier in the invite-only "GNUS Professional Network". Requires repeated successful engagements plus top-tier performance and team contribution.
3rd-party Security Audit: 95/100
Independent 3rd-party security audit firm (gold-tier scope, measured on a 2025 engagement)
Scored 95/100 on a Flutter app after implementing data protection, transport security, obfuscation, and CI/CD hardening end-to-end.
DORA Four Keys — Elite rating
Measured by client (GNUS, Inc.) across 5 repos (Jan 15 – Apr 15 2026, 90 days)
The app repo I lead alone rated Elite — 1.94 deploys/day, 0.6% change-failure rate. The 5-repo program overall rated High (3.0/day, 1.5% CFR, <1h MTTR).
06Company
A properly incorporated studio you can safely engage.
Company
Good Welchi, inc.
Representative
Yuta Kawai
Address
2-8-50 Setominami, Kochi, Kochi 781-0253, Japan
Capital
JPY 3,000,000
Founded
October 2, 2020
Key Clients
Keio University / GNUS, Inc. / VNOS, Inc. and others (alphabetical)
07FAQ
Commonly asked before getting in touch.
Yes. We contract as a Japanese corporation — invoicing, NDAs, and standard confidentiality agreements are all supported.
1–5 days / week, primarily remote. Rates depend on role, commitment, and length of engagement — please reach out.
Lead Engineer, Tech Advisor, main app engineer, or solo-lead on small projects. Strongest slot: app-side lead.
Yes — BFF (Spring Boot), Firebase/Cloud Functions, Unity, TypeScript all have production mileage. If the app needs it, we do it.
Yes. We support formal contracts and NDAs as a corporation.
08Contact
Feel free to get in touch.
Weekdays 10:00–19:00 (JST)
We aim to reply within two business days.