Stefan Eckhardt doesn’t think much of pitch decks. He’d rather get a well-written email. And when a working prototype arrives in a founder meeting, what interests him isn’t what it does — it’s what it tells him about how the founders think.

Eckhardt is managing director of Callisto Health, the Berlin-based health-tech investor that led February 2026’s $4.5 million round into YOU(th), with caesar., adesso Ventures, Antler, Moonstone and 1024 Ventures alongside, and angel cheques from Alan founder Jean-Charles Samuelian and HomeToGo’s Patrick Andrae. YOU(th) is the German smartphone-screening startup whose app turns face videos, voice recordings, eye and skin images, typing patterns and step data into more than fifty digital biomarkers across ten-plus organ systems. The whole check-up takes under two minutes.

Blood-based diagnostics are accurate but expensive and infrequent. The wearable-and-app layer is cheap and continuous and harder to validate. Callisto’s thesis is that the phone in your pocket is going to eat into both ends — by becoming a screening device that runs passively in the background of normal phone use, rather than one you remember to open.

That thesis is more interesting once you know what Eckhardt thinks is wrong with the rest of the category. His diagnosis of the longevity-app market is that it is full of products that collect data, return a score, and never produce the one thing that brings users back: an insight the user couldn’t have arrived at on their own. Plenty of founders are building the dashboard. Not many of them are offering answers.

What made you decide to back YOU(th)?

Basically, most of the decision is around the team. We met the team and believed they were capable of building a great company in the area they are working in.

How much of your decision is based on what they’ve already done versus your belief in what they can achieve in the future?

They had already built a prototype, which helps a lot. A prototype allows you to understand how founders work. You can see how they tackle problems, how they think about building a digital product, and how they deal with challenges like integrating partners. Having a working prototype, and ideally some customers, makes it much easier to have a productive discussion and assessment.

The assessment goes both ways. Investors assess the team, but the team also assesses the investor. It’s much easier to do that when there’s something real to look at rather than just slides and ideas.

Were you looking at other companies in the space?

Yes, we always look at the category before investing. We try to test products ourselves. In this case, we tested several products and also spoke to potential clients of YOU(th) to understand whether building something like this made sense.

At a basic level, everyone understands that mobile phones are always with you and that the sensors are improving. Once people understand that, many will say it’s a cool idea to assess biomarkers more frequently using a phone — especially if it becomes passive. Even spending a minute or two a day measuring something is interesting.

That was enough for us to think this was a strong team working on a good application.

What did you see as the long-term opportunity for the product?

What the team will likely build in the coming months is strong use cases — either for consumers, insurance companies, or clinics. The key is to build habit-driven use cases so people come back regularly and perceive the biomarker tracking on their phone as a real value-add.

I often say that lifestyle apps work when they give you insights you couldn’t have just felt yourself. If you get a meaningful insight, you’ll come back again and again. What doesn’t work well is when you input lots of data and only get a score back without real insight.

Did you have particular competitors in mind when you looked at the space?

We tested a few products, including [Binah AI, an Israeli company developing video-based vital sign measurement]. There were two or three others we tried as well. The goal was to gather as much information as possible, test the models, and understand both the accuracy and the limits of the technology.

Do you see this as competing with things like blood biomarker platforms or AI health companions?

Everyone in the space has a different angle. Blood biomarkers are definitely more reliable, but you can’t measure them very frequently.

Then you have data integrators. For example, I tried [InsideTracker, a US company that combines genetics, blood biomarkers and activity data]. It gives great insights and recommendations. But once you learn what you need to do, you may stop opening the app as often.

My belief is that something like YOU(th), which sits on top of those data sources and provides more frequent insights, might encourage users to come back more regularly.

For example, I use Garmin and Oura. I chose Oura because I work in this field and wanted to test it. But as a runner, cyclist and triathlete, Garmin or Polar give me the information I instinctively want. That’s how I differentiate between tools.

Did the economics of a smartphone-based solution play a role in your decision?

Yes. A digital product where you own the models could theoretically have very high gross margins. That’s definitely attractive.

That said, it doesn’t mean we would never invest in a company like [Function Health, the US blood-testing platform]. They simply operate at a different stage of the customer journey.

What do you look for most in founders?

We look for people who are driven to work as a team. Ideally, people who believe in collaborating closely and working together physically to build the product.

We also prefer founders who are product-driven or problem-driven rather than just solution-driven.

It’s not about one specific profile. It depends heavily on the business model. In some of our other companies we try to find founders who fit our internal culture. But in venture investments across different industries, founders can build different cultures and still succeed. We just need to understand their path to success.

If someone approached you cold, what would they need to show to get a meeting with you?

A clear 30-second elevator pitch. They should explain that they have a solution that solves an existing problem more easily for customers — or cheaper — than existing solutions. And they should explain why they believe their team is capable of building that business over the next five years.

What constitutes a red flag for you?

One thing I look for in the first meeting is whether founders actually listen. I try to see how they manage the discussion.

I also imagine how future employees or investors would perceive them. Would people want to work for this person? Would people want to invest in them? Someone can be very smart, but if the answer to those questions is no, it’s not a good investment.

When you look at a pitch deck, what are the most important elements for you?

Honestly, I don’t spend a lot of time reading decks. I usually flip through them quickly and then start asking questions.

I’d rather spend five or ten minutes looking at the deck and the rest of the time asking questions. If a deck is extremely superficial or overly complicated, that’s a problem.

Personally, I actually prefer a well-written email or a two-page document over a PowerPoint. Influenced by people like Steve Jobs and the idea of written narratives, I think two pages explaining clearly what you do can be much better than a slide deck.

Are there particular questions you always ask founders?

Yes, we tend to ask similar questions across most investments.

First, we try to understand the product from the customer’s perspective. We ask founders to explain the product clearly and explain what behaviour the customer needs to change in order to use it.

Second, we look at unit economics. Founders often explain unit economics, but we also want to understand how they plan to scale operationally from there.

Those two lines of questioning often reveal the most.

Is there an area of longevity you personally find most interesting?

I’ve personally experimented with many lifestyle interventions and supplements.

What I would really love to see is a product that gives truly personalised recommendations — maybe just two to five actions that are most relevant for you.

Right now, many solutions either overwhelm people or provide advice that people already know. I’d like to see something that asks a few questions, maybe about sleep, for example, and then gives a very small number of personalised steps.

For example, one personal experiment for me was magnesium L-threonate, [a form of magnesium that crosses the blood-brain barrier more readily than standard magnesium supplements]. My wife and I tried it, and it had a noticeable effect on sleep quality. A product that could guide someone to try something like that based on a few questions — and then gather feedback and iterate — would be very interesting to me.

Photography Hugo Barbosa