Example of an economy where Equilibria may not be efficient, where one agent is altruistic I'm looking for a theoretic example of an economy where one agent is altruistic, while the others are not, that would make a walrasian equilibrium not efficient.
This is an old question. The first example of inefficiency caused by altruism that I know of is due to Winter, S. (1969). A Simple Remark on the Second Optimality Theorem of Welfare Economics. Journal of Economic Theory, 1, 99–103, but I am sure others were around before that.
Now, the example in Winter(1969) does not answer your question because it features altruism for all agents in the economy. As mentioned by The Almighty Bob you can find another example in Heidhues and Riedel (2007). Another good reference is Dufwenberg, M., Heidhues, P., Kirchsteiger, G., Riedel, F., &amp; Sobel, J. (2011). Other-Regarding Preferences in General Equilibrium. The Review of Economic Studies, 78(2), 613–639, which is a summary paper of Heidhues and Riedel (2007) and contributions on the topics by the other authors.
Without more constraints on the preferences, it is not hard to find an example satisfying the conditions of your question. I am sure you could find it yourself, if you haven't already. But in order to make the answer complete here is one:

$\Omega \equiv \{(1,1),(1,1)\}$ (individual endowements)
$U_1 \equiv \min \{x_1,y_1\}$ (perfect complement, no altruism)
$U_2 \equiv \min \{x_2, y_2\} + 2*U_1$ (perfect complement, with (strong) altruism toward $1$)
Walrasian equilibrium = {(1,1),(1,1)}, together with whatever (finite) relative price you like.

I guess it is clear that the Walrasian equilibrium is not Pareto efficient. Notice that this &quot;result&quot; highly depends on the definition of the consumption space. If you add

Donation of good $x$ from $2$ to $1$, and
Donation of good $y$ from $2$ to $1$,

to the consumption space, then you would recover efficiency (although it might take a little care to define a meaningful notion of Walrasian equilibrium in this case).